I visited the path lab today to find out what is the minimum amount of blood you need in each bottle, and why. 7 useful facts about blood sample processing at your hospital lab:

1. The U&E tubes are centrifuged on arrival, and the gel at the bottom mixes with the plasma. This is why they need to be at least one third full to get U&Es, as below this there isn’t enough plasma that hasn’t been claimed by the gel, and it is only the plasma that can be used for U&Es.

2. FBC in the EDTA tubes can be done with only 0.2ml in theory, but this would require the lab doing it by hand rather than machine. This takes them time, so you will have to ask very nicely.

3. The coagulation tube must be fully filled to ensure the citrate in the tube is adequately diluted, or the INR will be spuriously high. Some labs offer smaller tubes with less citrate.

4. However, the d-dimer test works by binding the d-dimer products to a special antibody. This forms a solid mass. The amount of light transmitted through a certain volume of the blood sample when mixed with the d-dimer antibody is inversely proportional to the concentration of d-dimers. This means they can actually do the test with less than fully filled bottles.

5. Troponin needs a tiny amount of blood. Send whatever you can get for troponin; it will probably work.

6. TFTs need half a U&E bottle.

7. If the patient is difficult to bleed, use a paediatric tube, or ask the lab for their smaller bottles. They have reduced levels of all the EDTA/gel, so need less blood to get decent results.