What’s a blood draw?
Blood collection is the puncture of a vessel to obtain a blood sample. In most cases the puncture is done venously. A blood sample is usually taken as a diagnostic tool to examine various parameters in the blood, such as inflammation or coagulation values.
In rare cases it is also used as a therapeutic agent in the form of bloodletting. This is done, for example, in the clinical picture of hemochromatosis. In most cases, blood is taken from the upper extremity using special needles. In small children, blood can also be taken from the head or heel.
Which blood samples must be taken fasting?
There are only a few parameters that should be determined by taking a blood sample when fasting. These include, as the name suggests, the fasting blood glucose. This value is determined to exclude or confirm diabetes mellitus in patients.
A breakfast already served before taking the blood sample would falsify this value. A normal fasting blood sugar level is 70-100 mg/dl. The cholesterol value should also be determined with an empty stomach.
This value, which determines the fats in the blood, could also be falsified by a fatty breakfast before the blood sample was taken. A value that is too high could indicate a fat metabolism disorder. Normally the doctor should inform the patient beforehand whether he/she has to fast for the upcoming blood collection.
If there are any doubts, however, it is worth asking explicitly once again. It is also important to ask whether or not the patient should take their home medication before taking the blood sample. But what does fasting actually mean?
You should not have eaten anything in the last 8-12 hours before taking the blood sample. Drinking water, however, is allowed. Coffee and tea may also be drunk if they are not sweetened and drunk without milk.
Which blood samples do not have to be taken fasting?
Most blood values are not changed by the food ingested before a blood sample is taken. Therefore it is not important that the patient is fasting. The two exceptions to this are blood sugar and cholesterol.
As these two substances are part of our food, they can therefore be found in higher concentrations in our blood after a meal. A blood sample that is not fasting would falsify the values for cholesterol and blood sugar. Your doctor can tell you whether you have to be fasting for a blood sample or not.
Which tube is used for what?
When taking a blood sample, several blood samples are usually taken in plastic tubes with different coloured caps. This colour coding of the tubes has a meaning. The red tube contains the additive potassium EDTA.
This substance prevents blood clotting within the sample. The blood count, i.e. the individual cell types such as white and red blood cells, can be determined from this sample and values such as the red blood pigment (haemoglobin) can be determined. The red tube can also provide material for direct pathogen detection from the blood.
The brown tube contains a separating gel. This serves as a barrier between the liquid and solid components of the blood after centrifugation of the blood sample. Values such as liver and kidney values can be determined from the brown tube.
The blood salts (electrolytes) such as sodium, potassium and chloride can also be determined from this sample. The green tube contains the additive sodium citrate. This prevents blood clotting in the sample.
This enables the determination of coagulation values such as the INR and the Quick value. The violet tube also contains sodium citrate. The blood sedimentation rate is determined from this tube.
The white blood collection tube contains small plastic spheres. They activate blood clotting within the sample and thus serve to obtain blood serum as liquid blood components. Special values such as troponin can then be determined from this sample.
The yellow tube contains sodium fluoride. This additive inhibits the enzymes that are responsible for the breakdown of sugar. Therefore it is possible to determine blood sugar and lactate values from the sample without falsifying these values.
