Platelets are what help our blood clot. So if there are too many (“thrombocytosis”), there’s a risk of internal clotting, mainly a stroke. If there aren’t enough (“thrombocytopenia”), there’s a risk of internal bleeding (also a stroke!). But there are a lot of qualifications we have to put on this, namely 1) actual numbers, and 2) reason behind the abnormality. Normal number of platelets are around 150,000 – 400,000 per microliter of blood (some labs have 350,000 as high normal, others 450,000).
Thrombocytosis (too many platelets) — Strokes from abnormal clotting are not a risk until the number is over 1,000,000. That’s very uncommon. Also, it depends on the reason for the high number. Various diseases of the bone marrow (which produces blood cells), including cancers, can cause this, & that’s dangerous.
But the most frequent cause is “Reactive Thrombocytosis” — platelet counts rise in reaction to certain other conditions. Infection is the most common. Surgery or bleeding can also cause it, and anybody who had their spleen removed (usually from trauma) will have high platelet counts. In these cases, it’s harmless.
Thrombocytopenia (too few platelets) — Bleeding doesn’t occur until the platelet count drops to fewer than 25,000, Surgeons worry about operating, & dentists worry about pulling teeth, if platelets are under 50,000. But before worrying about anything, it’s important to repeat the test.
Some people have a harmless tendency for their platelets to clump together in the test tube. So the automatic counter thinks they’re something else, & doesn’t count them. Laboratories should routinely look for this under a microscope for anyone with low platelet counts, and make a note of its presence or absence. Sometimes they don’t.
In terms of the cause, there’s “Idiopathic Thrombocytopenia” (ITP), which can be dangerous. “Idiopathic” means “happens on its own” (or, “we don’t know what causes it”). There are various treatments to improve the counts; platelet transfusions can be given in emergencies. A key sign of bleeding from seriously low platelets are “petichiae” — tiny reddish-purplish spots on the skin, usually the legs, that do not turn white for a split second when pressing on them. They represent tiny bruises from blood that’s leaked out of the blood stream (from capillaries).
ALERT – if you ever have a rash with red spots, push on them to see if they turn white for a split second, before the color returns. That’s called “blanching,” most red rashes blanche. But if seemingly red spots don’t blanche, then they’re really petechiae, or if larger, “ecchymoses” (like bruises). Petechiae & ecchymoses not caused by obvious trauma may be more dangerous than many red rashes which do blanche.
More commonly, other conditions make platelet counts decrease. The most common is Cirrhosis of the liver. A low platelet count (perhaps 50,000 – 100,000) is usually the first sign. That’s not dangerous in terms of bleeding, but it is in terms of liver disease.
A variety of inflammatory conditions (like Lupus) can cause low platelets, but not dangerous levels. Various medications can too, including alcohol. This can be dangerous if low platelets are due to medications used to treat & prevent blood clots (their names may be enoxaparin [Lovenox®] or dalteparin (Fragmin®]).
Another common cause is HIV, which causes platelet destruction in only some people because of a genetic tendency to get an immune reaction. I’ve had HIV patients with extremely low platelet counts, and for some unknown reason, they don’t tend to bleed. Treating HIV with standard medications corrects the platelet loss. If a patient without known HIV has low platelets, they need an HIV test (and tests for Hepatitis B and C as well)!