If the patient has a very low white blood cell count, the DNA yield may be insufficient to complete the analysis. In order to perform this test, both the patient's pre-transplant and the donor's specimen must be available. Specimens submitted for Molecular Diagnostics testing may contain substances that can inhibit PCR gene amplification. If an inhibitor is known to be present, attempts will be made to clear the specimen of inhibitor prior to amplification. Known inhibitors include heparin, medications and some levels of hemoglobin or IgG. The major inhibitor we see in our laboratory is heparin; avoid drawing blood in a heparin tube or through a heparinized port. For molecular diagnostics testing, a lavender top (EDTA) tube is preferred. If specimens containing heparin must be submitted, note "heparin" on the requisition to alert the lab to 'clean' the specimen before analysis thus reducing turnaround time, improving retention of DNA in the specimen and increasing our ability to return a conclusive result.