What does the bleeding time really measure?

Here’s a very good question about the diagnostic use of the bleeding time.

Q. I’m currently studying heme for boards and came across a practice questions that used platelet count, bleeding time, PT and PTT values to differentiate between certain diseases/problems. I was just wondering how in both Vitamin K deficiency and liver disease you can get an increase in PT and PTT but the bleeding time doesn’t change…I guess I figured that bleeding time would have to increase.  Can you explain this to me?

A. Yeah, that does sound weird, you’d think the bleeding time would change – but actually, the bleeding time is only a measure of platelet function. It really has nothing to do with coagulation!

I kind of think of it like this: the platelet plug is the first thing to form, and that is enough to stop the bleeding from the incision made at the beginning of the test. The coagulation cascade happens next, and the status of that won’t be apparent in the bleeding time results. The patient might have some more bleeding later if their coagulation system is really screwed up…but the bleeding time assay will be done by then. In reality, it probably happens a little more concurrently than that (platelet plug is followed very closely by fibrin formation – the two probably even overlap a bit), but I think it’s a good way to remember the concept.

The same reasoning fits with the way that people with coagulation factor disorders bleed (as opposed to patients with platelet disorders). People with platelet abnormalities tend to bleed spontaneously into mucous membranes without much provocation (probably because they’re having a hard time forming that initial platelet plug) whereas patients with coagulation factor abnormalities, like hemophilia, tend to have deep, severe bleeds that happen after some time has elapsed (because they form the initial platelet plug okay, but they can’t seal it up with fibrin very well, so they end up bleeding later on).