In 1963, the American physician and microbiologist Robert Guthrie introduced the dry blood test, the Guthrie test, with which he was able to diagnose the metabolic disorder phenylkenonuria (inability to break down the amino acid phenylalanine due to the lack of an important enzyme in the body) in newborns. Even today, this screening method is used worldwide, in which a few drops of blood from newborns are dripped onto a special filter paper. After drying the blood, the filter paper is placed on a nutrient-containing agar plate without phenylalanine and a specific type of bacteria is added. These particular bacteria can only multiply if there is a lot of phenylalanine in the dried blood drop. This is how it is determined if a newborn infant has the inborn error of metabolism and thus needs a special diet. Because detected early, these newborns can grow up normally on a strict phenylalanine-free diet without risk of mental abnormalities.
What is the dry blood test?
American physician and microbiologist Robert Guthrie introduced the dry blood test, the Guthrie test, in 1963 to diagnose the metabolic disorder phenylkenonuria in newborns. In later years, other factors in the blood have been identified for inborn errors of metabolism, so that today routine screening of newborns between the 36th to 72nd hour of life for certain metabolic disorders using the dried blood spot (DBS) test is standard practice. To collect a drop of blood with a special filter paper, newborns need only be pricked briefly on the heel. The dried filter papers are sent to selected special laboratories, where they are simultaneously tested for more than 30 metabolic diseases using complicated but efficient analytical procedures. Within hours or a few days, doctors and thus the parents receive the test results. For ethical reasons, newborn screening only covers diseases that are treatable if diagnosed at an early stage. Nowadays, newborn screening with the dry blood test is mandatory in many countries, but not in Germany. Nevertheless, this screening procedure for newborns is also used by many parents in this country and is financed by the health insurance funds.
Function, effect, and goals
The simplicity of blood sampling for the dry blood test led to the establishment of this screening method also for older children with other diseases to spare them the painful venous blood sampling with a needle. Nowadays, the DBS method is used in many areas of in vitro diagnostics (examinations outside the body by prior collection of blood, urine or saliva), including for adults. A small prick of the finger is sufficient to drop enough blood onto the special filter paper. For example, the concentration of vitamin D in the blood is determined in this way using the dry blood test. Low vitamin D concentrations indicate certain ailments. Even if the patient is still symptom-free at the time of the test, the attending physician can initiate therapy immediately. For therapeutic drug monitoring, where physicians need to know if the dose of a prescribed drug is properly adjusted in the blood, dry blood testing is used in part. For the DBS procedure, the physician can also give the utensils necessary for finger pricking and blood collection to a patient to take home. In this way, the patient can drip drops of blood onto the appropriate filter papers over a longer period of time and allow them to dry. He then brings them with him on his next visit to the doctor, or sends them directly to the designated laboratory. In this way, it is also determined whether a patient is taking his vital medications, such as antiepileptic drugs, correctly. In this context, the individual dose adjustment of immunosuppressants is a particular focus for the dry blood test. In order to set the correct concentration of drugs after an organ transplant, patients often have to have blood drawn at very short intervals. Here, another advantage of the DBS method becomes apparent, in that the already weakened patient is subjected to little stress during blood sampling.It is also practical that usually only a very small piece of the blood dried on the filter paper is needed for laboratory testing and therefore different tests can be performed from the same drop of blood. The “blood cards” can be stored 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 conspicuous in the past or not. In addition, this type of blood collection ensures greater safety for medical personnel from puncture injuries (possible transmission of infections is thus further minimized). Even laboratory personnel benefit from this type of testing, as it saves time and consumables in the pretreatment of the blood sample. Whole blood in tubes require extensive pretreatment, which is more time-consuming and costly.
Risks, side effects, and hazards
However, this type of blood collection also carries risks for later examination in the laboratory. In particular, if the filter papers are given to the patients to take home, it cannot be ruled out that they will use the utensils improperly, rendering the respective filter paper unusable. Furthermore, bacterial contamination or other dirt can lead to unusable test results. Scientific studies show that for certain parameters, such as some hormones, the test results can vary greatly between venous blood and the dry blood test. The reason for this is, among other things, the different amount of hematocrit (percentage of erythrocytes in the volume of blood) depending on the blood collection method. Therefore, many clinical studies are ongoing with dry blood tests to improve their reliability for certain parameters. Where necessary, analytical methods are adapted or blood from vein is recommended if required. Meanwhile, dried blood tests are also available for certain domestic or farm animals.
