Potassium
What it measures, and what changes the number.
Potassium is an electrolyte, a charged mineral dissolved in blood, that is involved in nerve signals and heart and muscle function. The test measures its concentration in serum or plasma, usually as part of a basic or extended metabolic panel.
It is typically read with sodium and chloride, the other main electrolytes, and with creatinine, urea and eGFR, because the kidneys control how much potassium leaves the body. Magnesium is sometimes checked too, since the two are closely linked.
Potassium is very sensitive to handling. Red cells hold much more potassium than the fluid around them, so any damage to cells during a hard draw (hemolysis), clenching the fist, a long tourniquet time, or delays before the sample is spun can raise the reported number. Serum and plasma results also differ slightly. Labs may label it Potasio, Potassi or K.
Often read alongside: Sodium, Chloride, Creatinine, eGFR (CKD-EPI), Magnesium