Antikoagulans

Ein Antikoagulans is the additive 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, and the serum tube is the exception that contains none of them:

• EDTA binds calcium and is used for the Blutbild and for HbA1c, because it preserves the shape of the cells.
• Citrate binds calcium reversibly, which is what coagulation testing requires, so a citrate tube has to be filled exactly to the mark, because 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 needs the sample to clot completely and then be centrifuged.

The consequence for a pharmacy is sharper than it first appears, because the wrong tube does not produce a slightly shifted value that someone can allow for. EDTA in a serum tube makes potassium too high and calcium too low, at numbers that have nothing to do with the person whose blood it is. An underfilled citrate tube lengthens clotting times, and neither result can be corrected afterwards, so the sample is rejected and the customer is asked to come back. That is why tube selection belongs to Präanalytik instead of 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 cause of Hämolyse. The additive also sets a time limit, because some parameters tolerate hours while others require centrifugation soon after the draw, which 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 and states the filling order and the volume, so a pharmacy planning venöse Blutentnahme asks for that list in writing before it starts.