Platelets

Platelets are not cells but cell fragments: your bone marrow makes giant cells that pinch off pieces of themselves, and those pieces last about a week before the spleen clears them. There are hundreds of billions of them, and their task is first aid at a damage. If a vessel wall breaks, they stick within seconds to the damaged spot and to each other, form a plug, and release substances that set clotting going and start the repair of the wall. The number is steered by a hormone from the liver, and the bone marrow responds to what the body asks. With an inflammation, an infection, blood loss or iron deficiency the bone marrow makes extra platelets, and that is a healthy response that falls again once the cause is gone. With a virus, alcohol or some medicines it temporarily makes fewer, and with a liver with scars a swollen spleen holds on to too many. Only below 50 does the chance of bleeding really increase; above that you usually notice nothing of an abnormal number. Your report prints the count in ×10⁹/L. Normal is 150 to 400 ×10⁹/L, and that is one of the few values where all four labs use exactly the same range. With a number below 50 the requester is called the same day, and one lab also calls above 700.

Platelets are in every blood count and say two things at once: whether your blood can clot, and whether your bone marrow is responding to something. A low count increases the chance of bleeding, from bruises and bleeding gums to, far below 50, spontaneous bleeding; a high count is usually not a problem of the platelets themselves but a sign that an inflammation, infection or iron deficiency is at play somewhere. The second reason is that the number belongs with lifestyle. Alcohol inhibits the bone marrow and breaks platelets down faster, and anyone who drinks daily often sits on the low side without anything else being wrong; a few weeks without drinking brings it back. Iron deficiency, through heavy periods or too little iron from food, lifts the count instead, and that is often the first clue before haemoglobin falls. The honest limitation is that a low number is most often false. Platelets can clump in the collection tube, so the counter counts too few; a repeat in another tube then shows a normal number. Only when it stays low, and certainly when haemoglobin and the white cells are low too, does the question turn to the bone marrow itself.

A low platelet count means fewer are being made, more are being broken down, or the counter counted too few. The most common explanation first: platelets clumping in the tube, which gives a falsely low number that disappears on repeat; a viral infection, which inhibits the bone marrow for a few weeks; alcohol, which inhibits the bone marrow and speeds up the breakdown; medication, such as some antibiotics, painkillers and blood thinners; a liver disease, where a swollen spleen holds on to too many platelets; and rarely autoimmune breakdown, where the immune system clears the platelets itself and only the count falls while the rest of the blood count is normal. A high count almost always means the bone marrow is responding to something. Usually it is the first and rarely the last: an inflammation or infection, where the platelets rise together with CRP; iron deficiency, often the first clue before haemoglobin falls; after surgery or blood loss, as the bone marrow replenishes the supply; and rarely a removed spleen, which no longer clears the platelets, or a bone marrow disease in which the bone marrow makes too many on its own. Above 700 a second look at the bone marrow belongs with it. Read platelets beside haemoglobin and the white blood cells, because three low lines at once point to the bone marrow and one low line does not, beside ferritin, which with a high number reveals an iron deficiency, beside CRP, which shows whether an inflammation is lifting the number, and beside MCV, which falls with iron deficiency.