A generic understanding is not enough for two of the lab tests we use: D-Dimer and Troponin. For these you need to know the actual assay available as well as the test characteristics of that assay. In this post, we’ll deal with D-Dimer.
We use the HemosIL Latex Immunoassay D-Dimer
The Good News: This is a very good test for excluding VTE. Here are the test characteristics:
Using the cut-off of 230 ng/ml, the test had a sensitivity of 100% for both PE and DVT. The specificities were 29.3-41.3% for PE and 33.8-41.6% for DVT (range b/c of two separate studies)
What does this mean to us? This test is sufficient to exclude PE in low or moderate risk patients as stratified by gestalt, Wells, or modified-Geneva criteria (moderate as per ACEP’s clinical policy).
The test is FDA approved for VTE exclusion (many hospitals use assays that are not).
Here is the product brochure for the HemosIL.
The Bad News: Since this test uses the 230 ng/ml cut-off, rather than the standard 500; we do not know if and how the new age-adjusted criteria can be used. While folks like Jeff Kline have said you may be able to half the published age-adjusted d-dimer levels with an assay like this, he is not sure. We really need a separate study.
The Weird News: SBH uses a clinical range that states normal goes up to 319 ng/ml, and will not show the test as abnormal until that cut-off. However if you go into comments, it appropriately lists the 230 number as the cut-off for VTE exclusion. Do not be fooled by this. The number validated for this assay is 230 ng/ml.