Here’s a great question from a reader:

Is it normal to have a prolonged bleeding time but a normal PTT?

This is a very good question because it makes you really think about what these tests measure. It also is one of my favorite types of questions: one that clears up some common, but rarely-discussed, misconception. We all have some little incorrect ideas lurking within the vast body of medical knowledge we’ve stuffed into our heads. These ideas seem so logical that they remain quietly in place until someone pokes at them with a challenging question.

So let’s look at this awesome question. Any time the bleeding time is prolonged, that isn’t normal. But I think what the question is really asking is: does it make sense to have an abnormal bleeding time and a normal partial thromboplastin time (PTT) at the same time?

The answer is yes, it does makes sense – because the two tests actually measure totally different things. We all know what the PTT measures: the intrinsic pathway (and final common pathway) of the coagulation cascade. That means factors XI, IX, X, V, II and fibrinogen. Fine.

The bleeding time is a different story (and this is where the rarely-discussed misconception comes in). The name (and the way the test is performed) makes it sound like the test should measure both the patient’s coagulation factors and platelet function. Just makes sense, right? Not true! The bleeding time ONLY reflects how the patient’s platelets act in vivo. It doesn’t have anything to do with how well the patient’s coagulation factors are working. That’s why patients with hemophilia have a normal bleeding time!

Just a side note: the bleeding time is not exactly the most reliable test – and so while we still learn about it in med school (and while it is still used in many hospitals), it is a test that is being phased out in favor of another test that uses a machine (called a platelet function analyzer) to measure platelet function. Not all hospitals have this new machine-based test – so the bleeding time is still performed in many places. But just be aware that there is quite a bit of variability in the test, and many people (rightly so) say that you should not use the bleeding time (assuming you have a different option).

Back to the question again. The normal PTT means that the patient’s intrinsic and final common pathways are working fine, and the prolonged bleeding time means something is wrong with the patient’s platelets. Patients with mild von Willebrand disease (so mild that factor VIII levels are not low enough to prolong the PTT) might have this combination of test results. Hereditary platelet disorders also prolong the bleeding time (but do not affect the PTT or INR).

The main thing to remember out of all of this is that coagulation tests measure coagulation factors, but the bleeding time only reflects platelet function. There. Glad we cleared that up!