May 31 2025
I (Geo) received this question on May 23, 2025: “I am using 100 mg of aspirin and 75 mg of Plavix. My PTT is 35.8 seconds; PT 13.8 seconds, INR 1.18; bleeding time is 1 minute and 30 seconds; clotting time is 8 minutes. Are these results normal when using the two drugs together?”
I answered that the questioner may wish to return to their provider when convenient and request current platelet function tests such as the PFA-100 or platelet aggregometry to monitor their dual platelet inhibitor therapy. Although time-honored, the BT is unreliable and therefore obsolete. The clotting time is also obsolete and has been replaced by the more accurate PTT. How would you answer this question? Please comment below.
Comments (3)
Antiplatelet Therapy
McGlasson DL, Strickland DM, Hare RJ, Reilly PA, Patterson WR, Evaluation of three modified ivy bleeding time devices, Lab Med:19, October 1988. This article was written a short time after the Surgicutt was marketed. It was considered an innovation in bleeding time devices. It was compared to the Organon-Teknika Simplate and Mayo Clinic Automatic Lancet. In 2001 the Institution I was working for stopped using the bleeding time. One exception however, was that the renal physicians still wanted to use the Surgicutt for screening subjects prior to a renal biopsy. In 2018 I was participating in a seminar at the University of Texas Health Science Center in San Antonio, TX. In a case study discussion the subject of the bleeding time result came up. I said “Seriously you all are still doing bleeding times?” The answer was “Yes we still do them for subjects undergoing a renal biopsy.” No one could give a reason why except “we are used to the data that screening device gives us.”
First, there is no need to monitor aspirin/plavix therapy unless there is some evidence of therapeutic failure. Second, the skin bleeding time (SBT) is considered an obsolete invasive test. A 1 min/30 sec SBT is very short and would most likely reflect a test failure–in other words a failed test result that would normally require repeat testing; but I would not recommend repeat testing of an obsolete invasive test. SBTs often fail due to poor operator technique or skin issues–especially a problem for aging patients. If you really want to know if the aspirin/plavix therapy is working, you could request a Multiplate assay, but unless you have evidence of treatment failure, you would most likely be better off spending the money on a nice meal!
To answer this question accurately, the normal reference ranges established by the local laboratory are required. Bleeding time and clotting time are historical assays that have largely been phased out in modern clinical practice due to their limited accuracy and reproducibility. However, defining these tests and outlining their normal reference ranges is still needed for interpretation. Today, a variety of platelet function analyzers are available to assess platelet activity more accurately. While I don’t have personal experience to endorse a specific system, some widely recognized platforms include:
PFA-100/200 (Siemens)
VerifyNow (Werfen)
Multiplate (Roche)
Thromboelastography (TEG)
ROTEM
AggreGUIDE / AggreDYNE
Chrono-Log Aggregometers
Sysmex coagulometers (specific models)
Flow Cytometry–based assays
The growing understanding of pharmacogenetics, particularly the role of genetic variation in determining individual response to antiplatelet therapy, underscores the importance of platelet function testing. It helps clinicians tailor treatment to maximize efficacy and minimize harm.