Plasma creatinine assessment in creatine deficiency: A diagnostic pitfall

Guanidinoacetate methyltransferase (GAMT) deficiency (creatine deficiency s yndrome) is a recently discovered inborn error of creatine biosynthesis. Af fected patients have elevated concentrations of guanidinoacetate, the metab olic precursor of creatine, in urine, plasma and cerebrospinal fluid. In ad dition, urinary creatinine excretion and plasma creatinine concentration ar e decreased. For biochemical evaluation of patients suspected to suffer fro m GAMT deficiency, correct quantification of creatinine in plasma is import ant. Here we report our experience with different quantification techniques . We found that creatinine in plasma from two GAMT-deficient patients appea red normal when measured by the Jaffe method but was decreased when measure d enzymatically or by HPLC. The apparently normal levels of creatinine as m easured by the Jaffe method were not caused by guanidinoacetate. In urine, the Jaffe method and the enzymatic method gave similar results, indicating that in urine no false elevations of creatinine can be expected. As the Jaf fe method is still widely used for routine plasma creatinine measurements, it is important to realize it cannot be used to exclude GAMT deficiency.