An Automated Enzymatic Method on the Roche COBAS MIRATM S for Monitoring Phenylalanine in Dried Blood Spots of Patients with Phenylketonuria

Objective: To describe and evaluate a phenylalanine dehydrogenase-coupled enzymatic assay for blood-spot phenylalanine (Phe) automated on the COBAS MIRATM S analyzer for monitoring Phe in phenylketonuria (PKU) patients, as part of a home testing program. Methods and Results: This method required a four-point calibration with each run and the useful range was 9.3 to 3500 μmol/L Phe. The within-run precision (CV%) was 8.8% at a mean of 77 μmol/L Phe and 5.3% at 787 μmol/L Phe. The between-run precision was 15% and 5.6% for 104 μmol/L and 748 μmol/L Phe, respectively. Blood-spot Phe determinations by this method were compared with plasma Phe determined by the Beckman System 7300 HPLC analyzer using 152 samples collected from PKU patients and 56 samples from patients without PKU. Linear regression analysis revealed the equationy = 0.933x + 14.9. The standard error of estimate (Sy.x) was 82.9 and the correlation coefficient (r) was 0.98. A positive bias, observed for the blood-spot Phe assay with specimens containing Phe concentrations below 200 μmol/L, was not due to carryover or tyrosine. Conclusion: The results indicate that this method is acceptable for monitoring blood Phe levels in PKU patients.