Q&A column

Editor: Frederick L. Kiechle, MD, PhD

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Q. Learning material from the past CAP continuing education course “Analysis of Synovial Fluid: A Joint Effort” included commentary about collecting and evaluating synovial fluid: “Certain anticoagulants, such as oxalate, powdered EDTA and lithium heparin, should not be used since they have ingredients that mimic crystals and can cause false positives or complicate the evaluation.”

There seems to be conflicting information in the literature concerning specimen preservative for crystal analysis. For example, the CLSI document titled “H56-A: Body Fluid Analysis for Cellular Composition; Approved Guideline” states: “Some texts indicate that lithium heparin and EDTA should not be used as anticoagulants, because they produce crystalline material that can be confused with pathologic crystals. However, others have used lithium heparin and EDTA without difficulty.” The text is followed by a table, “Specimen Requirements for Synovial Fluid,” that lists heparin and EDTA as the anticoagulants of choice for cell count, differential, crystals, and inclusions. It seems, according to the CLSI, both anticoagulants are equally acceptable or unacceptable.

The CAP Hematology and Clinical Microscopy Resource Committee’s Color Atlas of Body Fluids states, “Inflammatory fluids may clot spontaneously and, if an anticoagulant is used, sodium heparin is the agent of choice as other commonly used anticoagulants may introduce artifacts complicating crystal categorization.”

The third edition of Body Fluids: Laboratory Examination of Amniotic, Cerebrospinal, Seminal, Serous and Synovial Fluids states something similar to the Color Atlas of Body Fluids  : “For routine examination, the syringe used for removing the fluid should be moistened with an anticoagulant (approximately 25 units of sodium heparin per milliliter of synovial fluid). Oxalate, powdered ethylenediaminetetraacetic acid (EDTA), and lithium heparin should not be used because they can produce artifacts in the microscopic examination for crystals.”

One thing is certain: Oxalate is not acceptable. It is unclear, however, which preservative is recommended or if the recommendation is to not use a preservative. We follow the CSLI guideline and use lithium heparin for our crystal analysis. Should we use sodium heparin instead or no preservative at all? Is there a specific CAP recommendation that addresses crystal analysis preservative? If not, what does the CAP recommend?

A. Your quandary is certainly understandable, particularly in light of the sources you cite. While this issue can get confusing, keeping a few things in mind may help to clarify the appropriate approach for your laboratory.

First, a few specifics on the collection tubes. The references you cite recommend caution for oxalate, lithium heparin, and the powdered form of EDTA due to the risk that crystalline-like materials intrinsic to those types of collection tubes will be mistaken for clinically significant crystals during evaluation for crystals in synovial fluid. To my knowledge, the risk of false-positive crystal evaluation has not been reported for sodium heparin or liquid EDTA, and the references you cite seem consistent with that assertion, although the wording varies.