An article published by SK Hall et al at three hospitals in the United Kingdom in the June 2015 issue of Journal of Medical Screening, reported on an investigation into biases seen with dried blood spot (DBS) sampling in newborn screening laboratories. The paper is entitled “Newborn screening blood spot analysis in the UK: influence of spot size, punch location and haematocrit.” It describes how the location of the punch on a DBS card, spot size and hematocrit all affect analytical data from biomarkers of inborn errors of metabolism and related conditions.
The research team was able to conclude that differences in blood spot size, hematocrit and punch location substantially affected measured concentrations for analytes used in the UK newborn screening program. They reported in their article that this could affect false positive and negative rates. The researchers proposed that to minimize analytical bias, “these variables should be controlled or adjusted for where possible.”
There is currently a wide range of genetic and metabolic conditions that are screened for at birth, including congenital hypothyroidism, phenylketonuria, cystic fibrosis, Medium-chain acyl-CoA dehydrogenase deficiency (MCADD), and maple syrup urine disease (MSUD).
Since the early 1960s, early newborn screening has involved the collection of capillary blood, usually from a heel-prick, onto a DBS filter paper or card where the blood is dried and sent to a laboratory for tests.
The tests vary dependent on the condition being screened, but often they measure the build-up of a metabolite such as phenylalanine as observed in phenylketonuria patients. The DBS technique of collecting samples from newborns has eradicated the use of the more invasive and complicated venipuncture technique to collect blood.
To attempt to obtain a volumetrically precise sample, DBS cards are ‘hole punched’ to obtain fixed diameter sub-punches. In neonatal screening for example, 3.2 mm punches are popular, which yields about 3 µL of blood at 45% hematocrit (HCT). However, HCT can vary in newborns (from ~30 -65%) and this can have deleterious effect on results. This is because the actual volume of blood collected from the sub-punch can be very misleading due to the following analytical biases:
Another bias that can be observed, even if a fixed % HCT is applied, is spot area bias. This is because it is difficult to meter the volume of blood collected from a finger or heel directly to a DBS card so the actual diameter of the spot can vary the concentration of the analytes in the spot which, in turn, can bias the results.
To test the extent to which biases are observed in neonatal screening, SK Hall and co-authors of the study paper summarized here investigated to evaluate the following biases: spot spread bias, punch location bias, and spot area bias.
The impact that DBS has had on early detection of genetic diseases and errors in metabolism in newborn babies cannot be underestimated. However, the fundamental issues associated with attempting to quantitate from DBS filter paper or cards, especially in a cohort where HCT can vary wildly, demanded some improvements in the field of blood microsampling.
The need to overcome the limitations of DBS cards created an opportunity in the microsampling industry for new sample-collection devices that offered a more precise volumetric approach to blood sampling. Indeed, the volumetric nature of newer sample collection tools such as hemaPEN® and Mitra® devices based on VAMS® technology have the potential to overcome many of the issues (spot size, punch location and HCT spread biases) observed in the work cited here.
For example, the hemaPEN device offers end-users who are accustomed to using DBS cards with an easy transition to a next-generation precision DBS device. With a single heel-stick or finger-stick, the hemaPEN simultaneously collects 4 volumetrically identical (2.74 µL) blood samples via rapid capillary action.
Once the end-user has collected the samples and clicks the lid back onto the hemaPEN to shut it, those 4 samples are transferred onto 4 pre-cut DBS disks inside the device’s tamper-resistant housing. The hemaPEN now contains 4 blood spots of equal size with no HCT spread and is ready for delivery to the lab for DBS processing and analysis that eliminates the hole-punching step.
This article was summarized for our readers by James Rudge, PhD, Neoteryx Technical Director. This is curated content. To learn more about the important research outlined in this blog, visit the original article in the Journal of Medical Screening.
Image Credits: iStock, Neoteryx, Trajan
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