Blood collection

What is a blood draw?

The puncture of a vessel to obtain a blood sample is called blood sampling. In most cases it takes place venously. A blood sample is usually taken as a diagnostic tool to examine various parameters in the blood such as Inflammation values ​​or coagulation values. In rare cases it is also used as a therapeutic agent in the form of bloodletting. This occurs, for example, in the clinical picture of hemochromatosis.
In most cases, blood is drawn using special needles from the upper extremity. Blood can also be drawn from the head or heel of young children.

Read more on this topic at: Blood test

Which blood withdrawals have to be done on an empty stomach?

There are few parameters that should be determined by fasting blood sampling. As the name suggests, this includes fasting blood sugar. This value is determined to exclude or confirm diabetes mellitus in patients. Breakfast taken before the blood sample was taken would falsify this value. A normal fasting blood sugar is 70-100 mg / dl.

Read more on the subject at: Blood glucose testing

The cholesterol value should also be determined on an empty stomach. This value, which determines the fats in the blood, could also be falsified by a fatty breakfast before the sample is taken. Too high a value could indicate a lipid metabolism disorder.

Usually, the doctor should tell the patient beforehand whether he or she will need to be fasted for the upcoming blood draw. However, if there are any doubts, it is worth asking again explicitly. Another important question is whether or not home medication should be taken before the blood sample is taken.

But what does being sober actually mean? You should not have eaten anything in the 8-12 hours before the blood draw. However, drinking water is allowed. Coffee and tea may also be drunk if they are not sweetened and are drunk without milk.

Which blood samples do not have to be taken on an empty stomach?

Most blood values ​​do not change from the food ingested before a blood sample is taken. Therefore, it is not important that the patient be sober. The two exceptions to this are, on the one hand, blood sugar and cholesterol.Since these two substances are part of our food, they can also be found in higher concentrations in our blood after a meal. A blood sample that is not sober would falsify the values ​​for cholesterol and blood sugar.

The attending physician can tell you whether or not you have to be sober to have a blood sample.

Which tube is used for what?

When blood is drawn, several blood samples are usually taken in plastic tubes with different colored lids. This color coding of the tubes has a meaning.
The additive potassium EDTA is in the red tube. This substance prevents blood from clotting within the sample. The blood count, i.e. the individual cell types such as e.g. white and red blood cells, as well as values ​​such as the red blood pigment (hemoglobin) be determined. The red tube can also provide material for direct detection of pathogens from the blood.

There is a separating gel in the brown tube. After the blood sample has been centrifuged, this serves as a barrier between the liquid and solid components of the blood. 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 be determined from this sample.

The addition of sodium citrate is in the green tube. This prevents the blood from clotting in the sample. This enables the determination of the coagulation values, e.g. the INR and the Quick value.

The purple tube also contains the substance sodium citrate. The sedimentation rate will be determined from this tube.

There are small plastic balls in the white blood collection tube. They activate blood coagulation 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 breaking down sugar. It is thus possible to determine values ​​for blood sugar and lactate from the sample without these values ​​being falsified.

Is the order of the blood collection tubes important?

The order of the blood collection tubes plays a major role, since a wrong order can lead to certain values ​​being falsified.
The tubes should be removed in the following order: brown, green, red. The order of the other tubes does not matter.

The brown tube should be removed first, as prolonged congestion of the veins during removal can increase the potassium value and thus be falsified in the sample. The green tube should follow as the second one because you should make sure that it is full up to the calibration mark. This is important because the ratio between blood and additive in the tube is the same for determining the coagulation values. At the beginning of the blood collection, there is always a minimal amount of air in the tube of the collection system and this can prevent the green tube from filling up. The green tube should be mixed well after removal. This also applies to the purple tube.

Sources of error when taking blood

Some mistakes can quickly creep in when taking blood. It can happen that blood is drawn from the wrong patient or that patient samples are mixed up. In order to avoid this, it should be checked before each collection whether the information on the tube corresponds to the patient's name and date of birth.

A second source of error can be incorrect or inadequate disinfection of the puncture site. The sample can be contaminated by skin germs. Furthermore, if the arm is tied for too long when taking blood, incorrect values ​​can occur. In particular, the potassium increases.

Especially with the green tube, insufficient filling of the tube can lead to falsified coagulation values. This sample should therefore not be taken as the first tube at.

A final possible source of error in blood collection is the lack of mixing of the sample. This can lead to unwanted clotting of the blood and the sample can no longer be used.

Blood collection from the police - are they allowed to do that?

Blood can be taken by the police if you suspect you are able to drive or to determine blood alcohol levels at the time of the crime. However, there are some conditions that must be observed.

The blood collection may only be carried out by a doctor. The person concerned must not suffer any health disadvantages and he or she must consent to the investigation. In the absence of consent, the order of a judge is required. If the investigation could no longer be successful by waiting for the order, the public prosecutor's office or, above a certain rank, the investigating police officer can order the blood to be taken in exceptional cases. In this case, blood can be drawn against the will of the person concerned.

Is a blood sample assault?

Strictly speaking, taking a blood sample is a physical injury. It may therefore only be done with the patient's informed consent and subsequent consent. If the patient is no longer able to give his consent, the presumed will of the patient can be acted on, for example in the context of a serious accident, and the blood sample can be taken anyway.

Taking blood from children - these are the special features

Blood is drawn from children in the same way as from adults. However, there are some special features. Sometimes it is possible for children to take a capillary blood sample instead of a venous one. For this purpose, the heel of small children and the finger of older children are pricked and the blood is collected.

For taking venous blood from children, there are special needles that are much smaller in diameter than needles for adults. If children are particularly afraid or a difficult puncture is to be feared, the skin over the puncture site can be numbed with an Emla plaster. This patch should be applied about 15 minutes before the blood draw.

The veins on the head are often particularly suitable for taking blood from infants. This looks very brutal for the parents, but it can be less painful for the child than elsewhere. In addition, the veins on the head are clearly visible and accessible for the puncture.

Children often have to be restrained during the actual blood collection. This usually bothers them more than the actual pain during the puncture. It is important to calm the children down and also to position the parents near the child.

Taking blood from bad veins

Many patients hear when they see a doctor that they have bad veins. There are several reasons for this. Older people in particular often have veins that are not immediately visible to the doctor, be it due to cold extremities or a deeper location of the veins in the skin. Another reason for bad veins are so-called rolling veins, which roll away during the puncture.

If the vein conditions are poor, blood can still be taken. A tourniquet should be attached to expose the veins and, if the hands are very cold, it can help to wash them with warm water beforehand. Good lighting conditions should also be ensured.

The puncture site for blood collection should be carefully selected before a puncture is performed. Here it is often the case that a palpable vein may be more suitable than a clearly visible vein. If the patient has rolling veins, it can be helpful to tighten the skin over the vein as much as possible to prevent it from rolling away.

Bruise after blood collection - what to do?

A bruise does not necessarily appear after a blood sample has been drawn. Bruising after a blood sample is usually caused by not pressing the puncture site after removing the needle. The small hole in the vein has not yet been able to close and blood leaks into the surrounding tissue.

The body breaks down the blood on its own. Treatment of the bruise is therefore not necessary, you just need a little patience. However, this process can take a few days. Some home remedies that have a supportive effect are light cooling of the affected part of the body and substances such as arnica or aloe vera.

Read more on the subject at: How do you treat a bruise?

What is a butterfly?

The butterfly, also known as the winged cannula, is a special needle for taking blood. Butterflys are used in place of other needles in most hospitals and doctor's offices today. The butterfly consists of a thin cannula that has two flexible plastic wings on both sides. The cannula is connected to a tube to which the blood collection tubes can be connected. The two plastic wings are used for easier handling during puncture. The plastic housing that surrounds the needle can be pushed forward after the blood is drawn to secure the needle. This minimizes the risk of a needlestick injury.

Vacuum tube

There are two different systems for blood collection tubes. The vacuum tubes work with a negative pressure that is present in the tube. When taking blood, a specific adapter is attached to the puncture needle and the vacuum tube is placed on the adapter. Now the blood flows into the tube due to the negative pressure. A disadvantage of this system is that the negative pressure in fine veins can cause the veins to burst more quickly.