Choose the right tube according to the examination to be carried out:

Tube colours are standardised and meet French, European and international standards. Biochemical examination: use a dry tube (with or without separator gel) for a serum assay or a Heparin-Lithium tube for a whole blood or plasma assay. Assay on red blood cells (e.g. GSH-pxe, SODe): Whole blood is required to isolate the red blood cell. The dry (red) tubes contain coagulation activators, so the cells form a clot which prevents us from isolating the erythrocytes -> rejected tubes.

Choose the tube volume adapted to the species and the examination to be carried out:

The quantity of anticoagulant is calculated precisely according to the tube capacity (e.g. filling level not reached: dilution effect of the sample and disturbances of certain analyses due to too high concentration of anticoagulant).

Fill the sample tubes :

Sampling with a syringe: rapid and delicate transfer of blood into the tube, taking care to remove the needle from the syringe to avoid iatrogenic haemolysis of the sample. Gently invert several times to homogenize the blood with the anticoagulant. Recommended order if several tubes are to be filled: Dry without additive, Citrate, Dry with additive, Heparin and EDTA. This minimizes clotting-related interference.

Avoid interference due to incorrect choice of anticoagulant:

Lithium Heparin allows all the biochemical analyses, electrolytes and acid-base balance to be carried out. Caution: Sodium heparin obviously interferes with Na+ assays. – Citrates do not allow either the assays of calcium, many enzymes, electrolytes and elements of the acid-base balance. – Fluoride tubes block glycolysis: essential for blood sugar dosing. – EDTA is a chelating agent; it traps metals -> impossible to dose Zn, Cu, Fe, Ca, Mg,… EDTA chelates calcium and as such is to be avoided for biochemical assays in general. Indeed, a large number of biochemical assays use calcium as a cofactor and are therefore strongly disturbed: Mg, Creatinine, PAL, Amylase for example will be strongly underestimated. Conversely, interactions with electrolyte assays will give results of K+ and Na+ strongly overestimated.

Lipemia, hemolysis and jaundice are three main pre-analytical conditions that may skew the result.

Lipaemia (= lactescence)

It is caused by the presence of triglycerides. It is a change that is most often post-prandial and physiological (= non-pathological). It can be avoided by fasting for 12 hours. Some results may be distorted (e.g. Na+ and Cl- in the ionogram, measured by indirect potentiometry).
Hemolysis

Most often hemolysis is a strict in-vitro change, caused by a difficult to take blood sample / inappropriate needle diameter, a delay in separating the serum (or plasma), accidental freezing or overheating of whole blood, a delay in sending/receiving the sample, and finally lipemia (weakening of the red blood cells). There are several mechanisms that will falsify certain test results during hemolysis: Changes in absorbance (e.g. creatinine, bilirubins, lipase, amylase, cholesterol, triglycerides. The degree of hemolysis required to alter the result may vary. Interference and direct measurement of hemoglobin (e.g., total protein, ALT) Release of molecules of diagnostic interest or involved in the chemical reactions of the analyzer (e.g., potassium, folate, LDH, AST, iron and phosphorus)

Icterus

An icteric sample often already reflects a pathological condition and has as its main consequence an alteration of absorbance which will distort the measurement of several analytes. This is particularly true for creatinine, which can be lowered falsely.

Light

Vitamins A, E and D3 and β-carotene degrade in light. -> protect the tube with e.g. aluminium foil.

Sample mixtures

Only a few methods have been validated to accept mixed samples. Sometimes, these sample mixtures are accompanied by a reserve e.g. NF U47 600-1 specifies that “the result obtained from a mixture of samples does not predict the results that would be obtained from individual analyses”. Mixing of samples may result in a loss of sensitivity of the method by dilution, by reaction in the manufactured mixture, or by camouflage due to contact with samples from animals not having the same status with regard to the disease sought.