Dry blood test
Even today, this screening method is used worldwide in which a few drops of blood are dropped on a special filter paper by newborns. After drying the blood, the filter paper is placed on a nutrient-containing agar plate without phenylalanine and given a certain type of bacteria.
These particular bacteria can only multiply if much phenylalanine is present in the dried blood drop. Thus, it is determined whether a newborn infant has the congenital metabolic disease and thus needs a special diet. Because early recognized, these newborns can grow up with a strict diet without phenylalanine normal growth without the risk of mental deformities.
What is the dry blood test?The American physician and microbiologist Robert Guthrie introduced in 1963 the dry blood test, the Guthrie test, with which he was able to diagnose the metabolic disorder phenylkenonuria in newborns.
In later years, other factors in the blood have been identified for congenital metabolic diseases, so that nowadays the routine screening of newborns between the 36th to 72nd hour on certain metabolic disorders with the dry blood test (DBS) is standard.
For catching a drop of blood with a special filter paper, the newborn need only be piked on the heel for a short time. The dried filter papers are sent to selected specialized laboratories, where they are now being tested on more than 30 metabolic diseases at the same time as complex but efficient analytical methods. Within hours or a few days doctors and thus the parents receive the examination results. For ethical reasons, only diseases that are diagnosed at an early stage are treatable as part of the neonatal screening.
Today, in many countries, the newborn screening with the dry blood test is obligatory, but not in Germany. Nevertheless, this screening procedure for newborns is also used by many parents in Germany and financed by the health insurance funds.
Function, effect & goals
The ease of taking blood for the dry blood test meant that this method of examination was also established for older children with other diseases in order to spare them the painful venous blood collection with the needle. Today, the DBS method is used in many areas of in vitro diagnostics (out-of-body examinations by previous collection of blood, urine or saliva), including for adults.
A small finger tap is enough to drip enough blood on the special filter paper. For example, this determines the concentration of vitamin D in the blood using the dried blood test. Low levels of vitamin D indicate certain conditions. Even if the patient is still symptom-free at the time of the examination, the treating physician can immediately initiate the therapy.
For therapeutic drug monitoring, where doctors need to know if the dose of a prescribed drug is properly adjusted in the blood, some dry blood tests are used. For the DBS procedure, the doctor can also give the utensils necessary for fingerstroke and blood collection to a patient home. This allows it to drop drops of blood on the appropriate filter papers and allow to dry over a longer period of time. Afterwards, he brings them with him to the next doctor's visit, or sends them directly into the intended laboratory. In this way, it is also determined whether a patient is taking his vital medication, such as antiepileptic drugs properly.
In this context, the individual dose setting of immunosuppressants is a particular focus for the dried blood test. In order to adjust the proper concentration of the drugs after organ transplantation, patients often need to be bled at very short intervals. This shows another advantage of the DBS method in that the already weakened patient is only slightly burdened with taking blood.
It is also practical that only a very small fraction of the blood dried on the filter paper is required for the laboratory examination and therefore different examinations from the same blood drop can be carried out. The "blood cards" can be kept clean, dark and cool for many years. Thus, if desired, it can be checked even after a long time, whether a certain parameter in the blood was already conspicuous in the past or not. In addition, this type of blood collection provides more security for the medical staff from puncture wounds (possible infection transmissions are thus further minimized).
Even the laboratory staff benefit from this type of examination because it saves time and consumables when pretreating the blood sample. Whole blood in tubes must be pretreated consuming, which is more time consuming and expensive.
Risks, side effects & dangers
However, this type of blood collection also poses risks for later examination in the laboratory. In particular, if the filter papers are given to the patient home, can not be ruled out that they use the utensils improperly and thus the respective filter paper is unusable.
Furthermore, bacterial contamination or other dirt leads to useless test results. Scientific research has shown that for certain parameters, such as some hormones, the test results between venous blood and the dried blood test can vary widely. The reason for this is among other things the different amount of the hematocrit (portion of the erythrocytes at the volume of the blood) depending on blood drawing procedure.
Therefore, many clinical trials are running dry blood tests to improve their reliability for certain parameters. Where necessary, the analytical methods are adjusted or, if necessary, blood from the vein is recommended. Meanwhile, there are dried blood tests for certain domestic or farm animals.