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Urinalysis & Body Fluids

Collection & Handling

We’ve just explored the sophisticated physiology of synovial fluid. Now, we must focus on the practical, and absolutely vital, steps required to get a representative sample of that fluid from the patient’s joint to our microscope

The procedure to collect synovial fluid is called arthrocentesis. It is an invasive procedure, a sterile joint tap performed by a clinician, often an orthopedist or rheumatologist. The evidence they extract - the synovial fluid - is the key to solving the mystery of the patient’s painful joint

Our job as laboratory scientists is to act as the forensic specialists. We must ensure that this evidence is collected, preserved, and transported in a way that maintains its integrity. A single pre-analytical error can contaminate the “crime scene,” destroy crucial evidence (like crystals or bacteria), or create misleading artifacts. There is very little room for error

Arthrocentesis: The Procedure

While we don’t perform the tap, understanding the basics helps us anticipate potential issues

  • Sterile Technique: The procedure is performed under strict sterile conditions to avoid introducing bacteria into the joint, which could cause a devastating iatrogenic septic arthritis
  • Volume: The amount of fluid collected can vary dramatically, from less than 1 mL in a normal joint to over 50 mL in a large, inflamed effusion. We must be prepared to work with whatever volume we receive
  • Bedside Observations: The clinician should note the ease of aspiration and the initial appearance and viscosity of the fluid. A normal, viscous fluid will be difficult to aspirate and will form a “string” as it is pulled into the syringe. A low-viscosity inflammatory fluid will flow easily

Tube Selection: The Critical Triumvirate

Because we need to perform tests across multiple laboratory disciplines (microbiology, hematology, chemistry), a single tube is not sufficient. The fluid should be allocated into a specific set of tubes, each with a designated purpose

Ideal Tube Allocation

  • No Anticoagulant (Red Top or Plain Sterile Tube)
    • Department: Chemistry / Gross Analysis
    • Primary Tests: Appearance, Viscosity (“String Test”), Clot Observation, Glucose, Protein, Uric Acid, LDH
    • Rationale & Critical Considerations
      • This is the priority tube for observing viscosity and clot formation
      • A spontaneous clot is a significant pathological finding indicating inflammation, as normal fluid lacks fibrinogen
      • Using a tube without anticoagulant prevents interference with chemical assays
  • Sodium Heparin (Green Top)
    • Department: Microbiology
    • Primary Tests: Gram Stain, Bacterial Culture
    • Rationale & Critical Considerations
      • This is the priority tube for microbiology and must be collected aseptically
      • Heparin is the preferred anticoagulant as it does not inhibit bacterial growth
      • CRITICAL: Do not use EDTA or Sodium Fluoride, as they are bacteriostatic (inhibit bacterial growth)
  • EDTA (Lavender Top)
    • Department: Hematology / Microscopy
    • Primary Tests: Total Cell Count, WBC Differential, Crystal Analysis
    • Rationale & Critical Considerations
      • This is the priority tube for cell counts and crystal analysis
      • EDTA is the best anticoagulant for preserving cell morphology
      • CRITICAL: A powdered EDTA anticoagulant can create crystalline artifacts that interfere with crystal analysis, potentially leading to a misdiagnosis. Liquid EDTA is preferred

Crystal Conundrum: Powdered EDTA vs. Liquid EDTA

This is a critical pre-analytical pearl

  • Powdered EDTA: The small, crystalline particles of powdered EDTA are birefringent under polarized light and can be mistaken for pathogenic crystals (MSU or CPPD). This can lead to a devastating false-positive diagnosis
  • Liquid EDTA: This is the preferred anticoagulant for the hematology tube if crystal analysis is a possibility. The EDTA is already in solution and will not create crystalline artifacts
  • Sodium Heparin (Green Top): This is an acceptable alternative for crystal analysis if a liquid EDTA tube is not available. While cell morphology is not as well-preserved as in EDTA, it will not produce confusing crystal artifacts

Bottom Line: For any joint tap, the ideal collection includes a plain red top, a green top (or other sterile tube for micro), and a liquid EDTA lavender top

Handling & Transport: Urgency & Temperature

Synovial fluid is a STAT specimen. Analysis should be performed as soon as possible after collection

Why the Urgency?

  1. Cell Lysis: WBCs will begin to degrade, falsely lowering the cell count. A delay can make a septic joint appear less inflamed than it truly is
  2. Crystal Instability: MSU (gout) and CPPD (pseudogout) crystals can change over time
    • Changes in pH and temperature can cause crystals to dissolve or precipitate in vitro
    • Phagocytized crystals may be released from cells as they degrade
    1. Decreased Glucose: Glycolysis by cells and bacteria will falsely lower the glucose level

The Ideal Protocol

  • Timing: The specimen should be delivered to the lab and analyzed within 1-2 hours of collection
  • Temperature: Transport and store at room temperature
    • Do NOT refrigerate.: Refrigeration can cause the precipitation of cryoglobulins and other crystals, leading to artifacts and false positives
  • Prioritization: If testing must be delayed, the priorities are:
    1. Microbiology culture setup
    2. Cell count and differential slide preparation
    3. Crystal analysis

Specimen Rejection Criteria

While we try to work with every precious sample, there are times when a specimen’s integrity is too compromised to yield a reliable result

  • Clotted Specimen in an Anticoagulated Tube: A specimen received in an EDTA or heparin tube that is clotted is unacceptable for cell counts. The cells are trapped in the clot, and any count on the remaining fluid will be falsely low. The lab must note that the specimen was clotted and that the cell count cannot be accurately performed. The presence of the clot itself, however, is a reportable and significant finding
  • Improper Tube for Requested Test
    • Request for culture on an EDTA tube
    • Request for crystal analysis on a powdered EDTA tube (should be noted with extreme caution)
  • Mislabeled or Unlabeled Specimen: As with any specimen, this is an absolute cause for rejection

Conclusion

The pre-analytical phase of synovial fluid testing is a minefield of potential errors that can have profound clinical consequences. A false-positive crystal identification can sentence a patient to a lifetime of unnecessary medication. A falsely low cell count due to a clotted sample can lead to a missed diagnosis of a joint-destroying septic arthritis. By championing proper collection tubes (especially liquid EDTA), insisting on rapid, room-temperature transport, and meticulously handling the specimen upon receipt, we lay the foundation for an accurate and clinically invaluable diagnosis. We are the guardians of the evidence