Defining dried blood spot diameter: implications for measurement and specimen rejection rates

Dried blood spot (DBS) specimen acceptance guidelines recommend rejecting specimens based on DBS size, often expressed as a diameter. Computer vision methods can estimate DBS size from images obtained from standalone equipment, smartphone cameras or existing laboratory instrumentation. However, no consensus definition of DBS diameter exists. We assessed how different DBS diameter definitions affect measurement and specimen rejection rates.

We compared computer vision estimates of DBS diameter on 1,991 DBS using two different calculation methods and on 22 DBS where paired images were taken from either side of the filter paper. We modelled the impact on specimen rejection rate in >163,000 DBS specimens.

Two different calculation methods for DBS diameter showed a mean difference <0.1 mm for circular DBS. Greater variability was observed for incorrectly applied DBS with a mean (standard deviation) difference of 0.29 (0.41) mm. DBS diameter measured from the front of the filter paper was approximately 0.41 (0.25) mm larger than from the back of the filter paper. Changing the DBS diameter definition could more than double the number of insufficient DBS (<8 mm), potentially leading to 4,000 additional repeat collections annually in the UK newborn screening programme.

DBS diameter definition can have a small but important and easily avoidable impact on measurement, impacting specimen rejection rates. We recommend that DBS diameter is defined as the diameter of a circle with equal area to the DBS, when measured from the opposite side of the filter paper to blood application.