Background: We describe a simple qualitative visual ultramicroassay based on the colorimetric method introduced by Heard et al. for the detection of biotinidase deficiency in dried blood samples spotted on filter paper.
Methods: The assay uses 3-mm discs of dried blood on Schleicher and Schuell 903 filter paper and ultramicrovolumes of each reagent. Ten thousand newborn samples from the National Screening Program for the Detection of Phenylketonuria were evaluated.
Results: The ultramicroassay shows a good reproducibility. The lower detection limit is around 2% of the mean normal activity. We found one sample with the absence of enzymatic activity, another that was between 10% and 30%, and 10 with activity levels <40%. There was coincidence of our results with those obtained by the conventional colorimetric method that uses B-PAB as substrate.
Conclusions: The qualitative colorimetric ultramicroassay does not require special laboratory equipment and it is suitable for the neonatal screening of biotinidase deficiency.