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D-dimer Reporting Discrepancies

Ms. Denise Broadbent, Laboratory Supervisor at Klickitat Valley Hospital in  Goldendale WA posted a question to Medlab-L I will summarize here, followed by a clear discussion from Warren Coffin of Crozer Keystone Health System:

Recently we ran a D-dimer, getting a result of >700 ng/mL (negative <230, normal up to 255 ng/ml).  The patient was transferred to another facility whose D-dimer result using a different system was 1.90 mg/L (negative <1.0, normal up to 2.6mg/l).  Understandably our ER doctor was concerned and has asked me to look into the matter.  Specimens were drawn the same day, several hours apart.  I and the hematology supervisor at the other facility have reviewed our QC records and repeated the specimens. Everything is fine except we called the D-dimer positive and the second facility called it suspicious.  I am aware of  the differences among instrumentation, reagents and methods, but, has anyone else seen a discrepancy like this before?

Warren Coffin’s edited discussion follows:

Ms. Broadbent’s institution obtained 700 ng/mL, equivalent to 0.700 mg/L. At the referral institution the result was 1.90 mg/L, equivalent to 1900 ng/mL.

What we must know to compare results is whether the systems are using D-dimer units or fibrinogen equivalent units (FEUs).  Assuming both systems are reporting D-dimer units, Ms. Broadbent’s result is 37% of the referral site’s.  This may indicate the disease had progressed by the time of transfer. However, 1 FEU = 2 D-dimer units, so if Ms. Broadbent’s method was reporting in FEUs and the referral site in D-dimer units the difference falls nearly within test variation. In either event, the elevation is enough to warrant repeat testing to monitor the patient’s condition.

There is an additional source of variation. Each D-dimer method employs a separate monoclonal antibody with different specificities, so it is not possible to compare results across platforms. Instead, you must compare institutional results to their current reference interval. However, as illustrated in this case, this does not always work.

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