Antikoagulans

Ein Antikoagulans is the additive sitting inside a blood collection tube that stops the sample from clotting once it has left the vein. Blood begins to coagulate within seconds of collection, so the tube has to intervene chemically, otherwise what reaches the laboratory is a clot with a little fluid around it. The colour of the cap indicates which additive is present, and the order in which the tubes are filled follows from that.

Three additives cover most of routine work. EDTA binds calcium and is used for the Blutbild and for HbA1c, because it preserves the shape of the cells. Citrate binds calcium as well, but reversibly, which is what coagulation testing requires, and a citrate tube depends on being filled exactly to the mark, since the ratio of citrate to blood is part of the method itself. Heparin inhibits thrombin and is used for plasma chemistry when a result is wanted without waiting for a clot to form. A serum tube contains no Antikoagulans at all, only a clot activator and often a separating gel, because that analysis wants the sample to clot completely and then be centrifuged.

The consequence for a pharmacy is sharper than it first appears. The wrong tube does not produce a slightly shifted value that someone can allow for. EDTA carried into a serum tube pushes potassium up and calcium down to numbers that have nothing to do with the person who was sitting in the chair. An underfilled citrate tube lengthens clotting times. Neither result can be corrected afterwards, so the sample is rejected and the customer is asked to come back, which is why tube selection belongs to Präanalytik rather than to the analysis.

Two further points matter in daily practice. Mixing is part of the additive working at all, so each tube is inverted gently the number of times the manufacturer states, and a tube shaken hard instead is a common route to Hämolyse. The additive also sets a clock, because some parameters tolerate hours while others require centrifugation soon after the draw, and that is a question for Probenlogistik before the first appointment is booked.

The laboratory decides which tube is used for which parameter, not the pharmacy. It supplies the tubes, states the filling order and the volume, and a pharmacy planning venöse Blutentnahme asks for that list in writing before it starts.