Platelets Flashcards
How much of our platelets are pooled in our spleens?
about 33%
What are the four major platelet tests?
count
bleeding time (or new alt)
blood smear morphology
platelet aggregometry
What are the four GENERAL causes fo thrombocytpenia?
artifactual
decreased production
increased destruction
increased sequestration
What are the two GENERAL causes of thrombocytosis?
primary (neoplastic)
secondary
What are some of the reasons you get artifactual thrombocytopenia?
EDTA-induced platelet clumping from the test tube
clotted specimen
platelet clumping in myelodysplastic or myeloproliferative disorders
How can you tell if the thrombocytopenia is artifactual/
the lab’s supposed to figure it out
check the perpheral blood morphology
check a citrated blood (light blue top tube) PLT count
What are some general causes of decreased PLT production?
decreased numbers of megakaryocytes due to aplastic anemia, viral infection or drug-induced suppression
or impaired productoin of platelets by megakaryocytes due to a myelodysplastic process or megaloblastic process
What are some classic drugs that suppress megaK production?
chemo agents thiazides lasix alpha interferon quinine sulfonamides cimetidine/ranitidine vancomycin
ETHANOL
WHat are some immune causes of increased platelet destruction?
ITP neonatal purpura post-transfusion purpura drug-induced immune thombocytopenia HIT autoimmune disorders
What are some non-immune causes of platelet destruction?
increased loss (using them all up)
increased use due to pathologic clotting (DIC)
sequestration (hypersplenism)
What causes immune thrombocytopenic purpura (ITP)?
presumed due to anti-platelet antibodies
associated with viral infections and autoI disorders
If you suspect ITP and you want to treat it as such (with steroids or rhogam or rituximab), what do you get first?
the bone marrow biopsy
because all the drugs could affect the bone marrow. obviously get a smear before that
What is the most common cause of severe neonatal thrombocytopenia?
fetomaternal platelet incompatibility
What type of antibody accoutn for 90% of the cases of neonatal purpura in caucasians?
human platelet antigen 1a antibodies
if mom is HPA1a negative, and the baby daddy is likely positive (because negative is rare), then mom will make antibodies against baby’s platelets to cause a severe thrombocytopenia
How do you make the diagnosis of neonatal purpura in caucasians?
mom’s serology
mom and dad’s platelet antigen testing
genotype parents and baby
How do you treat neonatal purpura?
Prenatal IVIg/G or transfuse infant with ABO compatible/HPA-1a negative platelets
Who gets post-transfusion purpura?
HPA-1a negative individuals
What is the factor will some Heparin bind that will lead to HIT?
platelet factor 4, which the platelet IgG will bind to and then you get the complexes that the body forms antibodies against
Which type of hepatin has the higher incidence of HIT?
bovine is a higher molecular weight, so seems to bind up more PF4
What percentage of patients who receive unfractionated (mixed) heparin will develop HIT?
2-20%
How do we diagnose HIT now?
ELISA for Heparin-PF4 complex
(has replaced the hep-associated PLT aggregation test because it does NOT require a heparin-free specimen)
can also do a serotonin release assay, but not the standard
What are the two forms of microangiopathic hemolytis anemias that lead to non-immune platelet destruction?
Thrombotic Thrombocytopenic Purpura (TTP)
Hemolytic Uremic Syndrome (HUS)
How do you diagnose non-immune peripheral plt destruction?
blood smear coags reticulocyte count LDH haptoglobin
What’s the cause of TTP?
either an acquired or congenital deficiency of ADAMTS13, which makes protein C unable to turn off, so you get platelet-rich thrombi throughout microvascular causing red cell fragmentation and clots
How can you diagnose TTP?
red cell fragments on smear thrombocytopenia high LDH low haptoglobin measure the ADAMTS13 level, activity assay and antibody levels
How do you treat TTP?
plasma exchange
What is the most common congenital disorder of platelet function
vWF def
What are causes of acquired disorders of plateet function?
ASA and NSAIDS
uremia
post-cardiac bypass pump
acquired vWF disease (seen with some myeloproliferative disorders like polycythemia vera)
How can we test platelet function?
Don’t use a bleeding time!
We have automatic function analyzers now that basically do an in vitro bleeding time
it’s the platelet closure time (PFA-100 is the most common analyzer)
The analyzers wont work if the platelet count is less than ___
90,000
How does platelet aggregometry work?
it measures platelet function by measuring platelet aggregation in response to platelet agonists like ADP, collagen and ristocetin
it’s expensive and labor itnesnive so rarely used
Why can’t you use the mean platelet volume?
Because platelets variably increase in size the longer they are in EDTA and no one has figured out a way to correct for this
