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Sheikh Sajjadieh M. Diagnostic value of polarized optical microscopy in pseudogout: A review of calcium pyrophosphate deposition disease in the clinical laboratory. mljgoums 2025; 19 (6) :1-4
URL: http://mlj.goums.ac.ir/article-1-1943-en.html
URL: http://mlj.goums.ac.ir/article-1-1943-en.html
Australian Institute of Medical and Clinical Scientists, Melbourne, Australia , mohammad_esfahan@yahoo.com
Keywords: Pseudogout, Calcium Pyrophosphate Deposition Disease, Chondrocalcinosis, Crystal Arthropathies, Synovial Fluid, Birefringence, Polarized optical microscopy
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To prepare a slide for synovial fluid analysis in the diagnosis of pseudogout, synovial fluid should first be collected via joint aspiration. If necessary, the sample can be centrifuged to concentrate the sediment. A drop of the concentrated fluid is then placed on a clean microscope slide, and a coverslip is gently applied. The specimen is examined under a polarized light microscope, ensuring proper alignment of the polarizer and analyzer. CPPD crystals, which are diagnostic of pseudogout, typically appear rhomboid or rectangular and exhibit weak positive birefringence: Blue when aligned parallel to the axis of polarized light and yellow when oriented perpendicular to it (11).
Differentiation from gout
Gout is characterized by the presence of monosodium urate (MSU) crystals, which are needle-shaped and exhibit strong negative birefringence under polarized light microscopy (Yellow when parallel and blue when perpendicular). Polarized optical microscopy enables direct visual differentiation between these crystal types, which is crucial for accurate diagnosis and appropriate treatment (10).
Importance of POM in the clinical laboratory
Polarized optical microscopy is a low-cost, high-impact diagnostic technique that is particularly valuable for identifying crystal-induced arthropathies such as pseudogout; however, its effective use depends on the availability of appropriate equipment - including a polarizing microscope with a compensator plate - and trained personnel capable of accurately recognizing crystal morphology. In addition, timely analysis of synovial or cerebrospinal fluid samples is essential, as delays can result in crystal dissolution or degradation, thereby compromising diagnostic accuracy. For laboratories that already possess a standard brightfield compound microscope, implementing POM for crystal analysis can be achieved affordably by adding a polarizing accessory kit. These kits, which typically cost approximately $100 - $150 USD, include a polarizer (Placed below the condenser) and an analyzer (Inserted into the eyepiece tube or microscope head). This simple modification allows existing microscopes to be adapted for the detection of birefringent crystals such as MSU and CPPD, which play a central role in diagnosing gout and pseudogout. For enhanced diagnostic accuracy, the use of a first-order red retardation plate improves crystal visualization under polarized light by facilitating differentiation between negative birefringence (MSU) and positive birefringence (CPPD). With careful slide preparation, appropriate illumination (Preferably LED), and basic training in interpretation, even modestly equipped laboratories can effectively perform crystal identification using POM, significantly improving diagnostic capability without the need for expensive instrumentation. Despite its diagnostic value, POM remains underutilized in routine laboratory practice, and greater emphasis on training and protocol development is needed to integrate it into standard arthritis workups.
Limitations and challenges
Accurate diagnosis of crystal arthropathies using polarized light microscopy can be challenging, particularly in chronic cases where crystal burden may be low and difficult to detect. Interpretation requires considerable skill and experience to distinguish true crystals from artifacts, and the risk of false-negative results increases if sample processing is delayed or if specimens are improperly stored, as crystals may dissolve or degrade over time.
Discussion
Accurate detection of synovial fluid crystals is essential for differentiating crystal-induced arthropathies, particularly for distinguishing gout from CPPD. Current evidence confirms that calcium pyrophosphate dihydrate crystals are the defining feature of pseudogout; however, their subtle morphology and weak birefringence continue to pose diagnostic challenges (1,2). The European League Against Rheumatism (EULAR) recommendations emphasize that microscopic confirmation of crystals remains a central requirement for establishing the diagnosis (1).
CPPD crystals typically appear as rhomboid or short rod-shaped structures with low birefringence, which contributes to their under-recognition in routine laboratory practice (5,7). Their optical behavior reflects underlying pathogenic mechanisms, including ANKH gene dysregulation and the accumulation of inorganic pyrophosphate within cartilage, which promote crystal formation and deposition (3). These characteristics underscore the need for meticulous synovial fluid examination, as weak birefringence may result in false-negative findings, particularly in samples with low crystal density or inadequate preparation.
Optimizing sample handling - such as gentle centrifugation to concentrate the sediment - and ensuring precise alignment of polarizing components during microscopy can significantly improve crystal detection (5,11). The importance of laboratory expertise and methodological validation has also been demonstrated in other immunological diagnostic settings, such as antinuclear antibody testing, where differences between microscopy-based and automated techniques substantially affect diagnostic sensitivity and specificity (12). Continuous competency training for laboratory personnel is therefore essential, as misidentification of MSU and CPPD crystals may lead to inappropriate clinical management (7). While MSU crystals exhibit a characteristic needle-shaped morphology with strong negative birefringence, CPPD crystals require greater interpretive expertise because of their weaker optical properties and variable morphology (4,11).
Emerging diagnostic technologies, including Raman spectroscopy, digital imaging, and Fourier-transform infrared (FTIR) spectroscopy, have demonstrated high accuracy in crystal identification (2,10). However, their high cost, technical complexity, and limited availability restrict widespread implementation in routine diagnostic laboratories. Consequently, polarized optical microscopy remains the frontline approach for synovial fluid analysis, particularly in resource-limited settings where rapid, accessible, and cost-effective diagnostic methods are essential.
Overall, the literature strongly supports the continued importance of polarized light microscopy as a practical and reliable method for detecting CPPD crystals in synovial fluid. Although advanced technologies may enhance diagnostic precision in the future, conventional microscopy - when performed under standardized conditions and interpreted by experienced personnel - remains the most impactful and widely accessible tool for confirming pseudogout (1,2,5,11). Strengthening laboratory procedures, optimizing sample preparation, and enhancing operator expertise will remain central to improving diagnostic accuracy and ensuring high-quality patient care.
Conclusion
Polarized light microscopy plays a pivotal role in the diagnosis of pseudogout by enabling direct visualization of calcium pyrophosphate crystals. Its implementation in clinical laboratories enhances diagnostic accuracy and facilitates differentiation between CPPD, gout, and septic arthritis. Wider adoption and standardization of POM within laboratory protocols, together with appropriate technician training, are essential for improving patient outcomes in crystal-induced arthropathies.
Acknowledgement
The author expresses gratitude to the clinical rheumatologist and laboratory colleagues for their valuable insights and constructive feedback during the manuscript's preparation, which significantly enhanced the clarity and relevance of the work. The author also recognizes the supportive contributions of academic resources and libraries that made accessing the referenced literature possible. Lastly, appreciation is given to the peer reviewers of the Medical Laboratory Journal for their thoughtful assessments and useful suggestions.
Funding sources
This research did not receive any specific grant from public, commercial, or not-for-profit funding agencies.
Ethical statement
The author confirms that this manuscript is original and has not been published or submitted elsewhere. No human participants, patient data, or biological samples were involved in the preparation of this review; therefore, formal ethical approval and informed consent were not required. All ethical principles related to scientific integrity, transparency, and research conduct have been fully observed. The author declares no conflicts of interest.
Conflicts of interest
The author declares no conflicts of interest related to this work.
Author contributions
Mohammadreza Sheikh Sajjadieh contributed to all aspects of the work, including the conception and design of the review, literature search and data interpretation, drafting of the manuscript, critical revisions for intellectual content, and final approval for the version to be published. The author takes full responsibility for all aspects of the work and affirms that all statements and conclusions made are backed by suitable evidence and references.
Data availability statement
This article is a narrative review. All data presented in this manuscript are derived from previously published studies, which are appropriately cited throughout the text. No new datasets were generated or analyzed. Additional information supporting the findings of this review is available from the corresponding author upon reasonable request.
Full-Text: (2 Views)
Introduction
Pseudogout, also referred to as calcium pyrophosphate deposition disease (CPPD), is a crystal-induced arthropathy characterized by the deposition of CPPD crystals within synovial fluid and articular cartilage (1). These crystals develop as a result of disturbances in pyrophosphate metabolism, chondrocyte dysfunction, age-related cartilage degeneration, and biochemical abnormalities within the joint microenvironment. Metabolic and genetic factors - including hemochromatosis, hyperparathyroidism, hypomagnesemia, hypophosphatasia, and familial CPPD syndromes - further contribute to its pathogenesis (2,3).
Once deposited, CPPD crystals trigger a strong innate immune response driven by activation of the NLRP3 inflammasome and neutrophilic infiltration, leading to acute inflammation of the affected joint (4). Clinically, pseudogout commonly presents as acute monoarthritis or recurrent inflammatory episodes, most frequently involving the knee. However, its manifestations may also mimic gout, osteoarthritis, rheumatoid arthritis, or septic arthritis, thereby increasing diagnostic complexity (3,5).
The prevalence of CPPD disease increases significantly with age and is widely recognized as being underdiagnosed, particularly in routine laboratory practice. Epidemiological studies indicate that CPPD accounts for approximately 4 - 7% of all arthritis cases and 20 - 30% of crystal-induced arthritis cases, with radiographic evidence of CPPD detectable in up to 15 - 20% of elderly populations (5,6). Nevertheless, radiographic chondrocalcinosis lacks both sensitivity and specificity, reinforcing the need for direct examination of synovial fluid for crystal identification.
Polarized optical microscopy (POM) remains the gold standard for definitive diagnosis, as it enables visualization of the characteristic rhomboid-shaped CPPD crystals exhibiting weakly positive birefringence (5-7). Despite its critical diagnostic value, POM is not routinely performed in many laboratories due to variability in technician training, workflow constraints, and insufficient standardization. Consequently, many cases of CPPD are either missed or misclassified as other forms of inflammatory arthritis (6,7).
Given these diagnostic challenges, there is a pressing need to standardize synovial fluid analysis protocols, improve technician training, and consistently integrate POM into laboratory workflows. Enhanced recognition and accurate identification of CPPD crystals will directly improve diagnostic accuracy and clinical management of pseudogout.
Methods
This narrative review aimed to provide a comprehensive synthesis of current knowledge on pseudogout, with particular emphasis on the diagnostic application of polarized optical microscopy (POM) in clinical laboratory settings. A systematic literature search was conducted using PubMed, Scopus, Google Scholar, and ScienceDirect to identify relevant articles published between 2000 and 2024. The search terms included “pseudogout,” “CPPD,” “calcium pyrophosphate crystals,” “crystal arthropathy,” “synovial fluid analysis,” and “polarized light microscopy.” The search encompassed original research articles, clinical guidelines, consensus statements, and review articles that provided insights into crystal morphology, diagnostic laboratory practices, pathophysiology, and the epidemiology of CPPD disease (1-11).
The selected articles were analyzed and synthesized qualitatively. Key themes were identified, including mechanisms of crystal formation, the role of metabolic disorders in disease pathogenesis, principles and techniques of POM, specimen collection and handling, laboratory workflow best practices, and factors influencing diagnostic accuracy. The findings were integrated to provide a structured overview that bridges clinical and laboratory perspectives.
Special attention was given to the pathophysiology of CPPD crystal formation. CPPD develops as a result of imbalances in pyrophosphate metabolism and chondrocyte dysfunction. Aging, prior joint trauma, and metabolic conditions such as hemochromatosis, hyperparathyroidism (8), and hypomagnesemia (9) contribute significantly to disease pathogenesis. In hemochromatosis, iron overload damages cartilage and increases extracellular inorganic pyrophosphate (PPi), thereby promoting calcium pyrophosphate crystal formation. Hyperparathyroidism elevates serum calcium levels, favoring crystal nucleation within degenerating cartilage, whereas hypomagnesemia diminishes magnesium’s inhibitory effect on CPPD formation and impairs PPi degradation, further facilitating crystal deposition. These metabolic factors underscore the importance of evaluating underlying systemic disorders in patients presenting with pseudogout.
The manuscript underwent internal peer review by the author, who holds a Master’s degree in Medical Laboratory Sciences and a PhD in Clinical Immunology, to ensure scientific rigor, methodological clarity, and relevance to laboratory diagnostics. In addition, an external clinical rheumatologist reviewed the manuscript to ensure clinical accuracy, contextual relevance, and applicability. Feedback from both internal and external reviewers was incorporated into the final version to produce a comprehensive and balanced review suitable for publication.
Results
Clinical presentation
Patients often present with acute monoarthritis, particularly involving the knee, wrist, or shoulder. Attacks are sudden and may be accompanied by redness, warmth, and joint effusion. Chronic CPPD can resemble osteoarthritis, with intermittent flares and progressive joint damage (1). Radiographic findings often reveal chondrocalcinosis, characterized by linear calcification within cartilage.
Laboratory DIAGnosis
Synovial fluid analysis remains the cornerstone of CPPD diagnosis. Key steps include:
Specimen collection and handling
- Synovial fluid should be aspirated under sterile conditions and examined promptly.
- EDTA tubes should be avoided for crystal analysis due to the potential for artifact formation. For accurate diagnosis of pseudogout through crystal analysis, synovial fluid should be collected in a tube containing sodium or lithium heparin, as this anticoagulant preserves the morphology and birefringence of calcium pyrophosphate dihydrate (CPPD) crystals observed using polarized optical microscopy. Heparin does not interfere with crystal identification, unlike EDTA, which should be avoided because it can dissolve calcium-containing crystals or create artifactual crystals that may lead to misinterpretation. If a cell count and differential are also required, a separate aliquot may be collected in an EDTA tube; however, for crystal evaluation, a heparinized tube or sterile plain container is recommended. Analysis should ideally be performed within 1 - 2 hours to ensure reliable results (11).
Polarized Optical Microscopy (POM)
Polarized optical microscopy is an essential diagnostic technique for detecting crystals, including monosodium urate (MSU) and CPPD, based on their birefringence - the property of splitting light into two rays. In this method, a drop of fresh synovial fluid or cerebrospinal fluid (CSF) is placed on a glass slide and examined under a microscope equipped with a polarizer and analyzer. A compensator plate (Usually red) enhances contrast, allowing MSU crystals to appear needle-shaped and negatively birefringent (Yellow when aligned parallel to the axis), while CPPD crystals appear rhomboid-shaped and weakly positively birefringent (Blue when parallel). Although this method is most commonly applied to joint fluid, rare detection of crystals in CSF may indicate unusual conditions, such as central nervous system involvement by crystal deposition. Accurate interpretation requires technical expertise and should always be correlated with clinical findings (Figure 1).
Pseudogout, also referred to as calcium pyrophosphate deposition disease (CPPD), is a crystal-induced arthropathy characterized by the deposition of CPPD crystals within synovial fluid and articular cartilage (1). These crystals develop as a result of disturbances in pyrophosphate metabolism, chondrocyte dysfunction, age-related cartilage degeneration, and biochemical abnormalities within the joint microenvironment. Metabolic and genetic factors - including hemochromatosis, hyperparathyroidism, hypomagnesemia, hypophosphatasia, and familial CPPD syndromes - further contribute to its pathogenesis (2,3).
Once deposited, CPPD crystals trigger a strong innate immune response driven by activation of the NLRP3 inflammasome and neutrophilic infiltration, leading to acute inflammation of the affected joint (4). Clinically, pseudogout commonly presents as acute monoarthritis or recurrent inflammatory episodes, most frequently involving the knee. However, its manifestations may also mimic gout, osteoarthritis, rheumatoid arthritis, or septic arthritis, thereby increasing diagnostic complexity (3,5).
The prevalence of CPPD disease increases significantly with age and is widely recognized as being underdiagnosed, particularly in routine laboratory practice. Epidemiological studies indicate that CPPD accounts for approximately 4 - 7% of all arthritis cases and 20 - 30% of crystal-induced arthritis cases, with radiographic evidence of CPPD detectable in up to 15 - 20% of elderly populations (5,6). Nevertheless, radiographic chondrocalcinosis lacks both sensitivity and specificity, reinforcing the need for direct examination of synovial fluid for crystal identification.
Polarized optical microscopy (POM) remains the gold standard for definitive diagnosis, as it enables visualization of the characteristic rhomboid-shaped CPPD crystals exhibiting weakly positive birefringence (5-7). Despite its critical diagnostic value, POM is not routinely performed in many laboratories due to variability in technician training, workflow constraints, and insufficient standardization. Consequently, many cases of CPPD are either missed or misclassified as other forms of inflammatory arthritis (6,7).
Given these diagnostic challenges, there is a pressing need to standardize synovial fluid analysis protocols, improve technician training, and consistently integrate POM into laboratory workflows. Enhanced recognition and accurate identification of CPPD crystals will directly improve diagnostic accuracy and clinical management of pseudogout.
Methods
This narrative review aimed to provide a comprehensive synthesis of current knowledge on pseudogout, with particular emphasis on the diagnostic application of polarized optical microscopy (POM) in clinical laboratory settings. A systematic literature search was conducted using PubMed, Scopus, Google Scholar, and ScienceDirect to identify relevant articles published between 2000 and 2024. The search terms included “pseudogout,” “CPPD,” “calcium pyrophosphate crystals,” “crystal arthropathy,” “synovial fluid analysis,” and “polarized light microscopy.” The search encompassed original research articles, clinical guidelines, consensus statements, and review articles that provided insights into crystal morphology, diagnostic laboratory practices, pathophysiology, and the epidemiology of CPPD disease (1-11).
The selected articles were analyzed and synthesized qualitatively. Key themes were identified, including mechanisms of crystal formation, the role of metabolic disorders in disease pathogenesis, principles and techniques of POM, specimen collection and handling, laboratory workflow best practices, and factors influencing diagnostic accuracy. The findings were integrated to provide a structured overview that bridges clinical and laboratory perspectives.
Special attention was given to the pathophysiology of CPPD crystal formation. CPPD develops as a result of imbalances in pyrophosphate metabolism and chondrocyte dysfunction. Aging, prior joint trauma, and metabolic conditions such as hemochromatosis, hyperparathyroidism (8), and hypomagnesemia (9) contribute significantly to disease pathogenesis. In hemochromatosis, iron overload damages cartilage and increases extracellular inorganic pyrophosphate (PPi), thereby promoting calcium pyrophosphate crystal formation. Hyperparathyroidism elevates serum calcium levels, favoring crystal nucleation within degenerating cartilage, whereas hypomagnesemia diminishes magnesium’s inhibitory effect on CPPD formation and impairs PPi degradation, further facilitating crystal deposition. These metabolic factors underscore the importance of evaluating underlying systemic disorders in patients presenting with pseudogout.
The manuscript underwent internal peer review by the author, who holds a Master’s degree in Medical Laboratory Sciences and a PhD in Clinical Immunology, to ensure scientific rigor, methodological clarity, and relevance to laboratory diagnostics. In addition, an external clinical rheumatologist reviewed the manuscript to ensure clinical accuracy, contextual relevance, and applicability. Feedback from both internal and external reviewers was incorporated into the final version to produce a comprehensive and balanced review suitable for publication.
Results
Clinical presentation
Patients often present with acute monoarthritis, particularly involving the knee, wrist, or shoulder. Attacks are sudden and may be accompanied by redness, warmth, and joint effusion. Chronic CPPD can resemble osteoarthritis, with intermittent flares and progressive joint damage (1). Radiographic findings often reveal chondrocalcinosis, characterized by linear calcification within cartilage.
Laboratory DIAGnosis
Synovial fluid analysis remains the cornerstone of CPPD diagnosis. Key steps include:
Specimen collection and handling
- Synovial fluid should be aspirated under sterile conditions and examined promptly.
- EDTA tubes should be avoided for crystal analysis due to the potential for artifact formation. For accurate diagnosis of pseudogout through crystal analysis, synovial fluid should be collected in a tube containing sodium or lithium heparin, as this anticoagulant preserves the morphology and birefringence of calcium pyrophosphate dihydrate (CPPD) crystals observed using polarized optical microscopy. Heparin does not interfere with crystal identification, unlike EDTA, which should be avoided because it can dissolve calcium-containing crystals or create artifactual crystals that may lead to misinterpretation. If a cell count and differential are also required, a separate aliquot may be collected in an EDTA tube; however, for crystal evaluation, a heparinized tube or sterile plain container is recommended. Analysis should ideally be performed within 1 - 2 hours to ensure reliable results (11).
Polarized Optical Microscopy (POM)
Polarized optical microscopy is an essential diagnostic technique for detecting crystals, including monosodium urate (MSU) and CPPD, based on their birefringence - the property of splitting light into two rays. In this method, a drop of fresh synovial fluid or cerebrospinal fluid (CSF) is placed on a glass slide and examined under a microscope equipped with a polarizer and analyzer. A compensator plate (Usually red) enhances contrast, allowing MSU crystals to appear needle-shaped and negatively birefringent (Yellow when aligned parallel to the axis), while CPPD crystals appear rhomboid-shaped and weakly positively birefringent (Blue when parallel). Although this method is most commonly applied to joint fluid, rare detection of crystals in CSF may indicate unusual conditions, such as central nervous system involvement by crystal deposition. Accurate interpretation requires technical expertise and should always be correlated with clinical findings (Figure 1).
.PNG) Figure 1. Polarized optical microscopy demonstrating needle-shaped monosodium urate crystals with strong negative birefringence, characteristic of gout (5,7,11): This image shows synovial fluid examined under polarized light microscopy with a first-order red (λ) retardation plate. Numerous needle-shaped crystals are visible, displaying strong birefringence with vivid color changes depending on their orientation relative to the polarized light axis. Crystals appearing yellow–orange when aligned parallel and blue when perpendicular to the slow axis are characteristic of monosodium urate (MSU) crystals, consistent with gout. The elongated, sharp morphology and intense negative birefringence help distinguish MSU crystals from calcium pyrophosphate dihydrate (CPPD) crystals, which are typically rhomboid and weakly birefringent. |
To prepare a slide for synovial fluid analysis in the diagnosis of pseudogout, synovial fluid should first be collected via joint aspiration. If necessary, the sample can be centrifuged to concentrate the sediment. A drop of the concentrated fluid is then placed on a clean microscope slide, and a coverslip is gently applied. The specimen is examined under a polarized light microscope, ensuring proper alignment of the polarizer and analyzer. CPPD crystals, which are diagnostic of pseudogout, typically appear rhomboid or rectangular and exhibit weak positive birefringence: Blue when aligned parallel to the axis of polarized light and yellow when oriented perpendicular to it (11).
Differentiation from gout
Gout is characterized by the presence of monosodium urate (MSU) crystals, which are needle-shaped and exhibit strong negative birefringence under polarized light microscopy (Yellow when parallel and blue when perpendicular). Polarized optical microscopy enables direct visual differentiation between these crystal types, which is crucial for accurate diagnosis and appropriate treatment (10).
Importance of POM in the clinical laboratory
Polarized optical microscopy is a low-cost, high-impact diagnostic technique that is particularly valuable for identifying crystal-induced arthropathies such as pseudogout; however, its effective use depends on the availability of appropriate equipment - including a polarizing microscope with a compensator plate - and trained personnel capable of accurately recognizing crystal morphology. In addition, timely analysis of synovial or cerebrospinal fluid samples is essential, as delays can result in crystal dissolution or degradation, thereby compromising diagnostic accuracy. For laboratories that already possess a standard brightfield compound microscope, implementing POM for crystal analysis can be achieved affordably by adding a polarizing accessory kit. These kits, which typically cost approximately $100 - $150 USD, include a polarizer (Placed below the condenser) and an analyzer (Inserted into the eyepiece tube or microscope head). This simple modification allows existing microscopes to be adapted for the detection of birefringent crystals such as MSU and CPPD, which play a central role in diagnosing gout and pseudogout. For enhanced diagnostic accuracy, the use of a first-order red retardation plate improves crystal visualization under polarized light by facilitating differentiation between negative birefringence (MSU) and positive birefringence (CPPD). With careful slide preparation, appropriate illumination (Preferably LED), and basic training in interpretation, even modestly equipped laboratories can effectively perform crystal identification using POM, significantly improving diagnostic capability without the need for expensive instrumentation. Despite its diagnostic value, POM remains underutilized in routine laboratory practice, and greater emphasis on training and protocol development is needed to integrate it into standard arthritis workups.
Limitations and challenges
Accurate diagnosis of crystal arthropathies using polarized light microscopy can be challenging, particularly in chronic cases where crystal burden may be low and difficult to detect. Interpretation requires considerable skill and experience to distinguish true crystals from artifacts, and the risk of false-negative results increases if sample processing is delayed or if specimens are improperly stored, as crystals may dissolve or degrade over time.
Discussion
Accurate detection of synovial fluid crystals is essential for differentiating crystal-induced arthropathies, particularly for distinguishing gout from CPPD. Current evidence confirms that calcium pyrophosphate dihydrate crystals are the defining feature of pseudogout; however, their subtle morphology and weak birefringence continue to pose diagnostic challenges (1,2). The European League Against Rheumatism (EULAR) recommendations emphasize that microscopic confirmation of crystals remains a central requirement for establishing the diagnosis (1).
CPPD crystals typically appear as rhomboid or short rod-shaped structures with low birefringence, which contributes to their under-recognition in routine laboratory practice (5,7). Their optical behavior reflects underlying pathogenic mechanisms, including ANKH gene dysregulation and the accumulation of inorganic pyrophosphate within cartilage, which promote crystal formation and deposition (3). These characteristics underscore the need for meticulous synovial fluid examination, as weak birefringence may result in false-negative findings, particularly in samples with low crystal density or inadequate preparation.
Optimizing sample handling - such as gentle centrifugation to concentrate the sediment - and ensuring precise alignment of polarizing components during microscopy can significantly improve crystal detection (5,11). The importance of laboratory expertise and methodological validation has also been demonstrated in other immunological diagnostic settings, such as antinuclear antibody testing, where differences between microscopy-based and automated techniques substantially affect diagnostic sensitivity and specificity (12). Continuous competency training for laboratory personnel is therefore essential, as misidentification of MSU and CPPD crystals may lead to inappropriate clinical management (7). While MSU crystals exhibit a characteristic needle-shaped morphology with strong negative birefringence, CPPD crystals require greater interpretive expertise because of their weaker optical properties and variable morphology (4,11).
Emerging diagnostic technologies, including Raman spectroscopy, digital imaging, and Fourier-transform infrared (FTIR) spectroscopy, have demonstrated high accuracy in crystal identification (2,10). However, their high cost, technical complexity, and limited availability restrict widespread implementation in routine diagnostic laboratories. Consequently, polarized optical microscopy remains the frontline approach for synovial fluid analysis, particularly in resource-limited settings where rapid, accessible, and cost-effective diagnostic methods are essential.
Overall, the literature strongly supports the continued importance of polarized light microscopy as a practical and reliable method for detecting CPPD crystals in synovial fluid. Although advanced technologies may enhance diagnostic precision in the future, conventional microscopy - when performed under standardized conditions and interpreted by experienced personnel - remains the most impactful and widely accessible tool for confirming pseudogout (1,2,5,11). Strengthening laboratory procedures, optimizing sample preparation, and enhancing operator expertise will remain central to improving diagnostic accuracy and ensuring high-quality patient care.
Conclusion
Polarized light microscopy plays a pivotal role in the diagnosis of pseudogout by enabling direct visualization of calcium pyrophosphate crystals. Its implementation in clinical laboratories enhances diagnostic accuracy and facilitates differentiation between CPPD, gout, and septic arthritis. Wider adoption and standardization of POM within laboratory protocols, together with appropriate technician training, are essential for improving patient outcomes in crystal-induced arthropathies.
Acknowledgement
The author expresses gratitude to the clinical rheumatologist and laboratory colleagues for their valuable insights and constructive feedback during the manuscript's preparation, which significantly enhanced the clarity and relevance of the work. The author also recognizes the supportive contributions of academic resources and libraries that made accessing the referenced literature possible. Lastly, appreciation is given to the peer reviewers of the Medical Laboratory Journal for their thoughtful assessments and useful suggestions.
Funding sources
This research did not receive any specific grant from public, commercial, or not-for-profit funding agencies.
Ethical statement
The author confirms that this manuscript is original and has not been published or submitted elsewhere. No human participants, patient data, or biological samples were involved in the preparation of this review; therefore, formal ethical approval and informed consent were not required. All ethical principles related to scientific integrity, transparency, and research conduct have been fully observed. The author declares no conflicts of interest.
Conflicts of interest
The author declares no conflicts of interest related to this work.
Author contributions
Mohammadreza Sheikh Sajjadieh contributed to all aspects of the work, including the conception and design of the review, literature search and data interpretation, drafting of the manuscript, critical revisions for intellectual content, and final approval for the version to be published. The author takes full responsibility for all aspects of the work and affirms that all statements and conclusions made are backed by suitable evidence and references.
Data availability statement
This article is a narrative review. All data presented in this manuscript are derived from previously published studies, which are appropriately cited throughout the text. No new datasets were generated or analyzed. Additional information supporting the findings of this review is available from the corresponding author upon reasonable request.
Research Article: Review Article |
Subject:
Laboratory Sciences
Received: 2025/05/13 | Accepted: 2025/12/18 | Published: 2025/12/21 | ePublished: 2025/12/21
Received: 2025/05/13 | Accepted: 2025/12/18 | Published: 2025/12/21 | ePublished: 2025/12/21
References
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