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).
Hi, just a short question, and I apologise because I may be nitpicking:
Wouldn’t the bleeding time be inclusive of the vasoconstriction bit of the “primary hemostatic response”? My lecturer has put down bleeding time as a blood vessel test, and not under the category of platelet tests (platelet count, blood film etc).
I know that VC does not contribute a lot to keeping bleeding time as short as possible, not as much as platelet aggregation anyway, but yup. Hope you can help clarify! (:
Thanks!
Good question! The bleeding time measures the time from the initial injury (the little lancet poke to the skin) to the formation of the platelet plug. Vasoconstriction happens immediately after an injury (prior to platelet plug formation) – so it is true that the bleeding time does cover that event as well. However, the test is used in clinical practice as a measure of how well the platelets work in vivo. It’s extremely rare to have a prolonged bleeding time when your platelets are functioning properly. In theory, you need vasoconstriction in order to help the platelets and coagulation factors bump into each other. In practice, though, as you correctly mentioned, there just aren’t many diseases in which the vasoconstrictive response is decreased to the point of affecting the bleeding time.
So: while in theory, abnormalities of vasoconstriction could prolong the bleeding time, in practice, we don’t see that very often, and the bleeding time is used simply as a platelet function test.
Thanks a lot for such a lucid explanation !
Thanks for a brief explanation, a lot help for my exam! 😀
Wonderful explanation.
Why bleed easily in cases of diseases involving coagulation? Our blood vessels are more fragile too?
Hi Rogerio –
In coagulation diseases, the coagulation system is not working properly, so the platelet plug cannot be sealed up with fibrin – so the patient bleeds. The blood vessels and platelets are usually fine – but the formation of fibrin is defective, which is what causes the bleeding.
I had a bleeding time test done prior to an FNA biopsy. I have to repeat the FNA procedure again after three months and have been told to repeat the bleeding time test again too prior to the FNA biopsy. Is it really necessary to repeat the bleeding time test? Can the results change in such a short period of time, or at all in the same person?
Thank you.
Hi Alison – In short: yes, the bleeding time could change over a short period of time, if you acquired some platelet problem (like you took a ton of aspirin, and the platelets were therefore not functioning well). The longer answer is that the bleeding time is not really a great test, because there is a lot of variability inherent in it. It’s fallen out of favor over the years (though it is still used in some places), and there is a different test, called the PFA-100 that looks at platelets in a similar way (but you take a blood sample and run it through a machine, and it is more standardized). Also, the bleeding time (as you no doubt read in the post) only measures platelet function (not fibrin formation). So if they have reason to be concerned about potential excessive bleeding, an INR and PTT should also be performed.
I’m serial commenting as I study, and just realised I’m leaving critical comments all over.
I love you site, your explanations are awesome, thank you very much for the resource!!! I would feel silly leaving a comment saying that after every post I read. I hope you don’t mind constructive criticism when I do see something that could be tweaked.
So useful to read this as I study for my year 2 medicine exams… thanks!
Excellent explanation, this is so well written and really helps students understand the logical processes behind pathology. Thank you!
Had the same Q. Great answer!
Just Joined….very lucid way of Explaining.
Thanks