Why have the tubes got different coloured tops?

Different coloured tops are frequently used when several tubes are used to take blood samples. The tubes will be coloured according to be substances that they must contain for the various blood components to be measured. Each substance has a different colour code.

Tubes are divided into 2 large groups:

• The first comprises «dry» tubes that do not contain any anticoagulant. Once a clot has formed containing fibrin, the tube is centrifuged. The analyses are then performed on the supernatant called the «serum».
• The second comprises tubes containing an «anticoagulant». Tests may in this case be carried out on the «whole blood»: or, following centrifuging, on the supernatant called the «plasma».

After tubes have been centrifuged what is the difference between plasma and serum?

• «Serum» is «plasma» without any «fibrinogen».
• In the presence of an anticoagulant, the fibrinogen will be in the supernatant «plasma».
• If coagulation occurs, the fibrinogen is enclosed in the clot in the form of fibrin and is no longer present in the supernatant, the «serum».

= Yellow tops
These contain the «serum» obtained after coagulation and centrifuging and on which virtually all clinical biochemical, infectious serology, hormonology, allergology and immuno-pathology analyses are performed.

= Purple tops
Mode of action:
This «non-liquid» anticoagulant (which eliminates the dilution factor encountered with a liquid anticoagulant) prevents the Calcium from ionising by chelation. Coagulation cannot occur in the absence of ionised Calcium.

Advantages:
Preserves red blood cells (RBC)and white blood cells (WBC) very well and enables platelets (PLT) to be counted by preventing their aggregation.
EDTA is the anticoagulant of choice for analyses and microscopic examination of «whole blood» in haematology and when studying haemoglobins (e.g. glycated haemoglobins!).

= Green tops
Mode of action:
Heparin, a natural anticoagulant which is also used therapeutically, inhibits coagulation by acting as an anti-thrombin and preventing the transformation of prothrombin (factor II) into thrombin (activated factor II).
Heparinised tubes are rarely used. They are useful when testing lymphocyte nuclei and for certain analyses (for example, of heavy metals etc.)

= Turquoise blue tops
Mode of action:
Sodium citrate neutralises Calcium ions by binding them to form a non-ionised complex (Calcium citrate)
10% Citrated tubes can be used to carry out virtually all analyses on haemostatic plasma (obtained after centrifuging) and to determine fibrinogen levels.

As it is a liquid anticoagulant liquid, it is important that the tube is correctly filled in order to respect the dilution coefficient.

= Grey tops
Mode of action:
Fluoride Tubes used to determine glycaemia.