Blood Draw - Pay Attention To This

Laboratory results: the blood sample

Blood tests are very important in medicine. Taking a blood sample in hospital is one of the first unpleasant experiences for the patient. But the family doctor will also often do one or the other blood test. Doctors have a great interest in the blood of their patients because this body fluid represents a mirror of the entire metabolism.

Therefore, the examination of the blood can provide important information about risk factors or diseases. In addition, the blood test is a possibility to rule out a large number of possible diseases with little effort, great speed and high cost efficiency (provided the corresponding laboratory values are normal), which is referred to as so-called laboratory diagnostic "screening".

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Before the blood draw

Before each blood sample is taken, the attending physician must carefully make a selection from the large number of laboratory tests available - there are actually thousands of different parameters in blood and other body fluids. This depends on the respective medical question (indication).

A blood sample can be drawn for the following reasons:

• Preventive medical check-up,
• Clarification (confirmation or exclusion) of a suspected diagnosis,
• Follow-up in case of confirmed disease,
• Assessment of the therapy success,
• Assessment of the further course of the disease (prognosis).

Many mistakes happen in the run-up to the laboratory analysis. These steps are known as “preanalytics”. The most common mistake of all is mixing up samples. This can happen if, for example, the tubes are labeled incorrectly when taking a blood sample.

A few points must be observed even before the blood sample is taken so that correct and coherent (plausible) test results are obtained in the end:

• When to take blood: ideally in the morning between 7 a.m. and 9 a.m.
• For certain examinations, the patient should be sober (e.g. blood sugar test): But being empty doesn't mean starving! The day before, nothing should be eaten after 8 p.m. Small amounts of water or unsweetened tea may be consumed on the morning of the blood test.

How much blood is drawn

When taking a blood sample, several tubes are almost always removed. The tubes are of different sizes and have colorful stubble. Depending on what and how many laboratory analyzes are to be determined, the laboratory needs a certain amount of blood. Approximately four to eight milliliters of blood fit into a collection tube. Even if ten tubes are removed, that is a maximum of half an “eight” of blood. Much smaller collection tubes are used for children or they are used with capillary blood. This is taken from the heel of infants. The patient should have time to rest (sit or lie quietly) for about a quarter of an hour before the blood sample is taken. The blood sample is then also taken while sitting or lying down.

The tubes marked with different colored stubble may contain additives. For some examinations, the blood in the tube must not clot and accordingly anticoagulant substances (heparin, EDTA or citrate) are added to these tubes. In other tests, the blood is supposed to clot and in these cases coagulation accelerators are added.

The most important blood collection tubes are:

Blood tubes with colored stubble © Rob Bowman

• Tubes with heparin (green stubble) or coagulation accelerator (red stubble): heart, liver, kidney, metabolism examinations (blood sugar, protein, fats, electrolytes, minerals, trace elements), vitamins, hormones, drugs, poisons etc.,
• Tube with EDTA (violet stubble): blood count, blood group, genetic tests etc.,
• Tubes with citrate (blue or black stubble): blood sedimentation, examination of blood clotting, etc.

In practice, the order in which the blood tubes are removed must also be observed. In particular, the blood coagulation tube should not be removed first.

After the blood draw

After the blood sample has been taken, the test material should be processed in the laboratory as soon as possible. The most common source of error here is incorrect or missing labeling of the tubes. This should be done before the blood sample is taken.

Handling of the blood tubes after collection

• The tubes must be filled correctly.
• Tubes with green, purple, blue or black stubble must be swirled after removal.
• Tubes with the red stubble must not be swirled.