HOW THE PAST MAY BRING THE FUTURE TO DISEASE DIAGNOSIS
where, given the right analytical technologies, dried blood assay could be just as effective as the common wet plasma assay.
“ So it can be argued that there are many assays ”
The success of the newborn screening initiative was its simplicity. However, one of the observations that soon became apparent was that even though DBS worked really well for semiquantitative assays like for PKU, the development of more quantitative assays became more of a challenge. Indeed, in recent years there have been efforts to address this by solving a number of fundamental quantitation issues with DBS. One such issue is volumetric hematocrit (HCT) effect, where the viscosity of the blood (due to percentage HCT) can bias the results when a sub-punch is measured from the DBS sample. This has been solved by the emergence of fixed volume sampling devices and also ‘whole spot’ extraction techniques. Another issue is blood plasma ratio effect. Compounds only found in the plasma fraction will be inversely proportional to the percentage HCT of that sample. This raises a fundamental debate in blood measurements as to what matrix we should measure our analytes in.
BLOOD VS PLASMA
I’ve been to a number of meetings where the question of blood vs plasma has been debated. Quite simply, which is the ‘better’ matrix to measure the physiological concentrations of a certain biomarker / drug? For simplicity, I’m going to refer to ‘plasma’ as plasma or serum. It must be noted that for some assays there are differences between plasma and serum, however I’m not going to address these in this article as these differences are often subtle. The big difference between plasma and blood is that plasma is clear and blood is not and this determines the type of analytical methodology.
There are certain cases where plasma is the only option. An example of this is in the measurement of plasma potassium. Potassium is actively pumped into cells so any degree of hemolysis will skew the data and the assay will fail. Furthermore, there are some incidences where it is vital that whole blood is measured and not plasma. One example of this is the measurement of the anti-rejection drug tacrolimus. The reason is that tacrolimus partitions primarily into blood cells. There are however plenty of assays where plasma or blood could equally be analyzed. One of these assays is the detection of hydroxylated vitamin D metabolites. These biomarkers are only found in the plasma fraction and not in the cellular fraction. To compare plasma and blood concentrations of the vitamin, either a measurement of the percentage HCT is needed for each sample or an average HCT value is taken and the observed concentration is corrected with respect to the HCT fraction. Alternatively, new reference ranges must be agreed on for blood. So it can be argued that there are many assays where, given the right analytical technologies, dried blood assay could be just as effective as the common wet plasma assay.
SO WHY PLASMA?
There are a number of arguments as to why many assays have been developed from plasma. The first and most important is the choice of detector. Many of the assays used in routine clinical analyzers today are conducted by measuring changes in the wavelength or intensity,
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