Purpose: The correct analysis of types and amounts of amino acids is highly important in the diagnosis of inborn errors of metabolism. The results obtained from heel blood drop and plasma of patients who were referred from primary health care institutions with pre-diagnosis of phenylalanine metabolism disorder were compared with liquid chromatography tandem mass spectrometry (LC-MS/MS) method.

Materials and Methods: The phenylalanine (Phe), tyrosine (Tyr) and Phe / Tyr measurements obtained from the dry blood drop sample and plasma were compared by the LC-MS / MS method.

Results: For the ≥108.49 μmol/L phenylalanine (Phe) cut-off value determined by dry blood drop sample, 97.3% sensitivity and 97.4% specificity were determined (AUC: 0.997, p <0.0001). For the ≥116.96 μmol/L Phe cut-off value determined by plasma, 97.3% sensitivity and 100% specificity were determined (AUC: 0.998, p<0.0001). In the Bland Altman analysis, plasma Phe values were higher than dry blood drop sample Phe values by an average of 1.3 μmol/L, while plasma tyrosine (Tyr) values were higher by an average of 0.5 μmol/L than dry blood drop sample values. There was no significant difference between Phe, Tyr and Phe/Tyr values obtained from plasma and dry blood drop sample in Passing Bablok regression analysis (p> 0.05).

Conclusions: In our study, similar Phe, Tyr, Phe/Tyr levels could be measured by LC-MS / MS method in 95% confidence interval from both plasma and dry blood drop sample. In the ROC analysis performed, the results obtained from the dry blood drop sample were thought to have sufficient specificity and sensitivity in clinical terms. It is thought that using LC-MS/MS method in newborn screening programs and following HPA cases may be beneficial.