Questions and answers on preanalytics

What is the difference between preanalytics and blood sampling?

Blood sampling is part of the pre-analysis.

The term "pre-analysis" covers even more, e.g. labelling, centrifugation and transport of the sample. In short: all processes that affect a sample before it is measured (see below for definition).

How long may the vein be congested?

Maximum 1 min.

If the vein is congested for a longer period, the sample becomes haemolytic and the red blood cells burst. The haemolysis falsifies many laboratory results, e.g. potassium, iron, GOT and potassium.

Is it absolutely necessary to observe the correct filling quantity for citrate tubes?

Yes, because otherwise the coagulation values are determined incorrectly.

The tube contains an anticoagulant (citrate), which is only effective if the blood volume reaches the correct fill level.

When is the patient fasting?

When he has not eaten for 12 hours.

When should a blood sample be taken?

If possible between 7:00 and 9:00 a.m.

The concentration of some lab values in the blood fluctuates throughout the day. However, the reference values or normal ranges are usually determined during morning blood sampling. A blood sample taken in the afternoon can therefore lead to misinterpretations.

What do I have to do if a sample can no longer be collected on the day of sampling?

You have to separate blood cake and serum by cenztrifigating.

Otherwise, substances from the different cells in the blood can enter the serum and falsify the laboratory results. A wrong potassium value would be typical here. If you do not use gel tubes, you must transfer the supernatant (= the serum) to another tube after centrifugation.

What do I have to do if I want to send in frozen serum/plasma?

Centrifuge the tube and transfer the supernatant into a second "neutral" tube. Close this tube, label it with a barcode and freeze it. Please do not freeze whole blood (the original tube). The laboratory cannot use such a frozen sample.

Please give the driver the sample in the safety bag for frozen samples. In this way the driver immediately recognizes that it is a frozen sample and transports it accordingly to the laboratory. If you place the sample in the "normal" safety bag, there is a great risk that the driver will overlook the frozen sample and it will arrive at the laboratory defrosted.

Is it possible to measure parameters from unlabelled/unglued tubes?

No. In this case the laboratory is not allowed to examine the sample.

The following also applies to blood grouping and antibody screening tests: The analysis tube must always be labelled with the name, first name and date of birth.

You can find more information on sample labelling here.

What happens if the sample labeling and the request form do not match?

The laboratory is not allowed to examine the sample.

Because it is not possible to clarify beyond doubt which patient the sample actually comes from. The risk of a possibly life-threatening misdiagnosis is incalculable.

Further information on how tu use request forms can be found here.

Can I also use serum tubes for glucose determination?

No, this is not recommended.

In serum, glucose is broken down by about 5% per hour (glycolysis) - even after blood collection. As a rule, the sample is not measured until several hours after taking the blood sample. During this time a considerable amount of glucose may have been broken down, so the measurement may give false low values. To avoid this, please always use tubes in which glycolysis is inhibited: Sodium fluoride (NaF) tubes or tubes with NaF and citrate (GlucoExact, Glucomedics).

Why are certain tubes required for the tests? Why can I not determine potassium from an EDTA tube?

The various additives such as citrate, NaF or EDTA make certain tests possible.

A blood count, for example, cannot be determined from a serum sample because it no longer contains any blood cells. For NaF see previous question (glucose determination). In addition, stabilizing additives in the tubes interfere with some tests: The ETDA tube contains additional potassium, which would be measured in a potassium determination from EDTA blood and thus lead to false elevated values.

What happens if the drug was taken immediately before the drug level was determined?

Too high a level will be measured and the dose of medicine may be incorrectly adjusted.

The target values for drugs in serum always refer to the so-called trough level. This is the lowest level in the body just before taking the medicine. Shortly after taking the medicine, however, the highest level is measured.