Synovial and Serous Fluid Analysis

Record the anatomic site, color, clarity, volume, visible clot, and other unusual features before centrifugation or treatment. Then divide the specimen according to the tests ordered and the receiving laboratory's validated procedure. No single tube sequence or fixed aliquot volume applies to every synovial or serous fluid.

Liquid ethylenediaminetetraacetic acid (EDTA) or sodium heparin may be suitable for selected cellular methods. A plain sterile container may be preferred for others. Oxalate, lithium heparin, powdered EDTA, glove powder, and other particulate material can resemble crystals or interfere with testing. Check the laboratory's procedure for which additives it accepts. Synovial specimens require prompt analysis because cells deteriorate and crystals can precipitate or change after collection. Refrigeration before crystal examination can create in vitro crystals.^{1,21 Keohane EM, et al. Rodak's Hematology. 2025.2 Jokic A, et al. Laboratory testing of extravascular body fluids. 2020.}

Body-fluid chemistry and automated cell-count methods require verification or validation for the specific fluid matrix, analyzer mode, and intended use. Under the Clinical Laboratory Improvement Amendments, a laboratory establishes performance specifications when a method lacks applicable manufacturer claims or is modified. Those specifications include accuracy, precision, analytical sensitivity and specificity, reportable range, reference intervals, and other characteristics needed for the test system.^{3-53 Clinical and Laboratory Standards Institute. Analysis of Body Fluids in Clinical Chemistry. 2018.5 Bourner G, et al. ICSH guidelines for the verification and performance of… 2014.}

The Body Fluid Cell Counts and Cerebrospinal Fluid lesson covers chamber selection, cytocentrifugation, analyzer limits, and morphologic review. The Manual Cell-Counting Calculations lesson covers counted volume, dilution factors, and duplicate counts.

Synovial fluid occupies movable joints and reduces friction between articular surfaces. Plasma ultrafiltrate crosses the synovial microvasculature, and synoviocytes add hyaluronate and other macromolecules. Type A synoviocytes have macrophage-like functions, and type B synoviocytes produce hyaluronate and matrix proteins. Hyaluronate gives healthy fluid its viscosity, and dissolved nutrients support avascular cartilage.^{1,21 Keohane EM, et al. Rodak's Hematology. 2025.2 Jokic A, et al. Laboratory testing of extravascular body fluids. 2020.}

Healthy synovial fluid is usually clear and colorless to pale yellow, has high viscosity, contains no pathologic crystals, and commonly has fewer than 200 WBCs/µL. Mononuclear cells predominate. Exact volume, protein, glucose, viscosity, cell count, and differential intervals vary with the joint, collection, method, and laboratory.^{1,21 Keohane EM, et al. Rodak's Hematology. 2025.2 Jokic A, et al. Laboratory testing of extravascular body fluids. 2020.}

Inflammation fragments hyaluronate and permits more plasma proteins and cells to enter the joint. The fluid may become cloudy, darker yellow, and less viscous. Blood can reflect trauma, anticoagulation, a vascular lesion, or hemorrhage into the joint. Uneven streaks of blood can support a traumatic aspiration. Appearance alone cannot establish the source.

The string test estimates viscosity by observing how far fluid extends before the strand breaks. Collection technique and temperature affect the result, and viscosity has limited diagnostic value. The historical Ropes mucin-clot test exposes hyaluronate to acetic acid. It is rarely useful for arthritis classification. Acetic-acid diluents also precipitate synovial mucin, so they are unsuitable for synovial cell counts.^{22 Jokic A, et al. Laboratory testing of extravascular body fluids. 2020.}

Synovial cell patterns divide joint disease into broad interpretive groups. The ranges overlap, especially between crystal arthritis and infection.^{2,62 Jokic A, et al. Laboratory testing of extravascular body fluids. 2020.6 Ravn C, et al. Guideline for management of septic arthritis in native joints… 2023.}

Compensated polarized-light microscopy is the main laboratory method for examining monosodium urate (MSU) and calcium pyrophosphate (CPP) crystals. Mix fresh fluid gently and examine it as a wet preparation. Ordinary light helps locate cells and crystals. Crossed polarizers show birefringence, and a first-order red compensator identifies the observed sign. Report the crystal type and whether the crystals are intracellular, extracellular, or both.^{2,7,82 Jokic A, et al. Laboratory testing of extravascular body fluids. 2020.7 Richette P, et al. 2018 updated European League Against Rheumatism evidence-based… 2020.8 Abhishek A, et al. The 2023 ACR/EULAR classification criteria for calcium… 2023.}

Synovial chemistry provides supporting evidence. Glucose is most useful when synovial fluid and blood are collected close together. In healthy fluid, glucose is commonly within about 10 mg/dL (0.6 mmol/L) of the paired blood result. Larger decreases occur in inflammatory and infectious disease, and the ranges overlap. Prompt separation and analysis limit continued cellular glycolysis. Protein rises in many inflammatory conditions and rarely identifies the cause. Synovial lactate can increase the probability of bacterial arthritis in selected settings. Assay-dependent cutoffs and limited sensitivity make it unsuitable as a stand-alone test. Uric acid may help when crystal examination is unavailable or inconclusive.^{22 Jokic A, et al. Laboratory testing of extravascular body fluids. 2020.}

When infection is suspected, testing commonly includes Gram stain and aerobic and anaerobic culture. The microbiology laboratory chooses containers, culture bottles, media, incubation, and organism-directed molecular tests. Selected gonococcal, mycobacterial, fungal, or Lyme investigations depend on clinical findings, epidemiology, and locally available validated assays. Laboratory evaluation for Lyme arthritis begins with serum two-tier antibody testing. Synovial PCR is an adjunct in selected cases.^{6,96 Ravn C, et al. Guideline for management of septic arthritis in native joints… 2023.9 Miller JM, et al. Guide to utilization of the microbiology laboratory for… 2024.}

The Cerebrospinal Fluid and Sterile Body Fluid Cultures lesson covers organism recovery, culture systems, special media, and molecular testing in normally sterile fluids.

The pleural, pericardial, and peritoneal cavities have parietal membranes lining the cavity walls and visceral membranes covering the enclosed organs. A thin fluid layer lubricates the opposed surfaces. Hydrostatic pressure, plasma oncotic pressure, membrane permeability, and lymphatic drainage determine how much fluid enters and leaves each cavity.^{1,101 Keohane EM, et al. Rodak's Hematology. 2025.10 Milevoj Kopcinovic L, et al. Laboratory testing of extravascular body fluids. 2020.}

An effusion can involve more than one mechanism. Appearance, cell counts, chemistry, cytology, microbiology, imaging, and the clinical setting each provide different information. Serous-fluid reference intervals are limited, and results require cavity-specific interpretation.

Pleural transudates usually reflect systemic pressure or protein disturbances. Exudates usually reflect disease involving the pleura. Light's criteria classify a pleural effusion as exudative when at least one criterion is met.^{1111 Roberts ME, et al. British Thoracic Society guideline for pleural disease. 2023.}

Collect paired blood and pleural fluid close together and use comparable methods. Light's criteria favor sensitivity and can label some diuretic-treated heart-failure effusions as exudates. In that setting, a serum-to-pleural albumin gradient above 1.2 g/dL can support a cardiac or transudative mechanism. Clinical findings remain part of the classification.^{1111 Roberts ME, et al. British Thoracic Society guideline for pleural disease. 2023.}

Fluid protein alone, specific gravity, cholesterol, bilirubin ratios, total WBC count, and appearance can support an interpretation. They are not part of Light's criteria. Reserve Light's criteria for pleural fluid. Pericardial fluid and ascites are classified differently.

Record pleural appearance before processing. Clear or pale yellow fluid can occur in a transudate or exudate. Turbidity may reflect leukocytes, organisms, cells, debris, or lipid. Centrifugation can separate cellular turbidity from persistent lipid-rich turbidity. Blood can arise from a traumatic collection, hemorrhagic effusion, or hemothorax. A pleural-fluid hematocrit above 50% of the peripheral-blood hematocrit supports hemothorax. Dilution and collection trauma affect borderline results.^{11,1211 Roberts ME, et al. British Thoracic Society guideline for pleural disease. 2023.12 Asciak R, et al. British Thoracic Society clinical statement on pleural… 2023.}

Choose further tests according to the fluid's appearance and the clinical question.^{10,1110 Milevoj Kopcinovic L, et al. Laboratory testing of extravascular body fluids. 2020.11 Roberts ME, et al. British Thoracic Society guideline for pleural disease. 2023.}

The normal pericardial cavity contains approximately 10 to 50 mL of fluid. Inflammation, infection, malignancy, trauma, uremia, autoimmune disease, systemic venous pressure, and impaired lymphatic drainage can produce an effusion. Rapid accumulation can compress the heart and contribute to cardiac tamponade.^{1313 Schulz-Menger J, et al. 2025 ESC guidelines for the management of myocarditis and… 2025.}

Pericardial protein, LDH, glucose, appearance, and cell counts have no universal intervals that reliably classify an effusion. Do not apply Light's criteria to pericardial fluid, because they can misclassify it. Testing follows the suspected cause and can include cell count and differential, bacterial culture, selected fungal or mycobacterial studies, and cytology. Neutrophil predominance supports acute inflammation or bacterial infection without proving the cause. Cytology can confirm malignant pericardial involvement. A negative result does not exclude it. Routine tumor markers and broad viral testing do not replace cause-directed evaluation.^{1313 Schulz-Menger J, et al. 2025 ESC guidelines for the management of myocarditis and… 2025.}

Ascites is excess fluid in the peritoneal cavity. Portal hypertension from cirrhosis is a common cause. Other causes include cardiac disease, malignancy, infection, pancreatic or biliary leakage, and lymphatic obstruction. Use the serum-ascites albumin gradient (SAAG), calculated from serum and ascitic samples collected at about the same time, to classify the pressure mechanism.^{1414 Biggins SW, et al. Diagnosis, evaluation, and management of ascites, spontaneous… 2021.}

SAAG = serum albumin − ascitic-fluid albumin

A high SAAG reflects portal pressure. It does not identify a specific organ disease. Ascitic total protein further narrows the likely cause. A high SAAG with protein above 2.5 g/dL supports cardiac ascites more strongly than uncomplicated cirrhotic ascites.^{1414 Biggins SW, et al. Diagnosis, evaluation, and management of ascites, spontaneous… 2021.}

The cellular threshold for spontaneous bacterial peritonitis (SBP) is expressed as an absolute PMN count, so a percentage alone cannot be used to apply it. A count of at least 250 PMNs/µL meets the commonly used threshold in the appropriate clinical setting. For a bloody specimen, a common correction subtracts one PMN for every 250 RBCs when the laboratory's policy permits it. Culture-negative neutrocytic ascites remains clinically significant.^{1414 Biggins SW, et al. Diagnosis, evaluation, and management of ascites, spontaneous… 2021.}

SBP occurs without a surgically treatable intra-abdominal source. Secondary peritonitis arises from a perforation, abscess, inflamed organ, or another intra-abdominal source. Multiple organisms, persistently high PMNs, low glucose, high LDH, high protein, imaging, and the clinical course can raise concern for a secondary process. This laboratory pattern warrants prompt communication to the clinical service. Culture specimens should be obtained before antimicrobial treatment when possible and routed according to the microbiology procedure.^{1414 Biggins SW, et al. Diagnosis, evaluation, and management of ascites, spontaneous… 2021.}

Appearance directs follow-up testing. Cloudiness can reflect infection, malignancy, chyle, or abundant cells. Milky fluid prompts triglyceride testing and, when needed, chylomicron testing. Green or brown fluid can prompt paired bilirubin testing for bile leakage. Ascitic bilirubin above serum, or a fluid-to-serum ratio above 1, supports leakage. Ascitic amylase supports a pancreatic or gastrointestinal source when interpreted with paired serum and imaging. Fluid creatinine above serum supports urinary leakage. When tuberculosis is suspected, testing may include ADA, mycobacterial culture, and a validated nucleic-acid test.^{10,1410 Milevoj Kopcinovic L, et al. Laboratory testing of extravascular body fluids. 2020.14 Biggins SW, et al. Diagnosis, evaluation, and management of ascites, spontaneous… 2021.}

Cytology can identify metastatic or gynecologic malignancy. Its limited sensitivity makes a negative result inconclusive. Macrophages, reactive mesothelial cells, lipid-laden cells, microorganisms, mucin, and laminated calcified psammoma bodies are descriptive findings that require correlation. Tumor markers such as carcinoembryonic antigen (CEA) and cancer antigen 125 (CA 125) have limited specificity and do not replace cytology, tissue sampling, imaging, or clinical assessment.^{1010 Milevoj Kopcinovic L, et al. Laboratory testing of extravascular body fluids. 2020.}

The PMN correction follows the general calculation principles in Manual Cell-Counting Calculations. Analytical methods for albumin, LDH, glucose, protein, lipids, bilirubin, amylase, and creatinine follow chemistry procedures. Culture, organism identification, and molecular testing follow microbiology procedures.