Evaluation of the diagnostic efficacy of enzyme colorimetric assay compared to tandem mass spectrometer in neonatal screening for phenylketonuria

Abstract

The present study aims at evaluating the diagnostic efficacy of an enzyme-based colorimetric method (Enzolve’s kit) for quantitative determination of phenylalanine in comparison to the gold standard tandem mass spectrometer (MS/MS). This study was conducted on 155 neonates aged 3–7 days attending the neurometabolic clinic of Cairo University Children Hospital as a part of the neonatal screening program. Samples of patients were selected according to their phenylalanine levels obtained by MS/MS. Group 1 included 49 normal neonates with phenylalanine level 150 μmol/L selected to cover the analytical range of the commercial kit. Blood samples on filter paper cards were reassayed by an enzyme-based colorimetric test kit. On correlating the phenylalanine level assayed using enzyme-based colorimetric technique using Enzolve kit and MS/MS technique, r = 0.83 and P < 0.01 were found. Also it yielded a regression equation; phenylalanine level assayed using Enzolve kit = (30.6 + 0.98 × phenylalanine level assayed using MS/MS). Upon detecting positive PKU cases using Enzolve’s kit at a phenylalanine cutoff level of 127 μmol/L and by MS/MS technique at a cutoff level of 150 μmol/L, a statistically significantly association (P = 0.000) and an agreement with kappa of 0.64 (P = 0.000) were produced. On constructing ROC curve for assay of phenylalanine using Enzolve’s kit, an area under the curve of 0.963 was found. The Enzolve kit is a simple ,cost-effective, rapid with an accepted overall accuracy test that can be used preliminary to the use of the more sophisticated and expensive MS/MS in screening as well as monitoring patients with PKU.