The diagnostic value of serum concentrations of 2-(alpha-mannopyranosyl)-L-tryptophan for normal renal function.

Background: We have previously reported that the serum concentration of 2-(alpha-mannopyranosyl)-L-tryptophan (MPT), tryptophan glycoconjugate, is a more accurate measure of renal function than that of serum creatinine concentration. The aim of the present study was to compare the diagnostic value of serum concentrations of MPT and creatinine as a measure of normal renal function.

Methods: A total of 156 subjects with serum creatinine concentration < or =1.60 mg/dL aged 0 to 88 years were recruited. Serum concentrations of MPT and creatinine, and creatinine clearance calculated by Cockcroft-Galt formula were determined. A diagnostic accuracy of serum concentrations of MPT and creatinine for normal renal function was analyzed by using receiver-operating characteristics (ROC) curves. In 82 subjects with normal renal function defined as calculated creatinine clearance > or =80 mL/min (aged 6 to 68 years), the correlations between age and/or urinary creatinine excretion, which is related to muscle mass, and serum concentrations of MPT or creatinine, were determined.

Results: In the ROC curve, the area under the curve (AUC) in serum MPT concentration was significantly greater than that of creatinine (0.855 versus 0.800, respectively, P < 0.001) and the cut-off levels associated with the greatest diagnostic accuracy were 90 ng/mL for serum MPT concentration and 0.70 mg/dL for serum creatinine concentration. The sensitivity, specificity, and positive and negative predictive values were 69.5%, 85.1%, 83.8%, and 71.6% for serum MPT concentration, and 53.7%, 81.1%, 75.9%, and 61.2% for serum creatinine concentration. A close correlation existed between serum creatinine concentration and age (r= 0.798, P < 0.0001) in 23 subjects aged 20 years or younger. Conversely, serum MPT concentration remained unchanged regardless of age (r=-0.135, P= 0.228). Furthermore, a close correlation existed between serum creatinine concentration and urinary creatinine excretion (r= 0.817, P < 0.0001), but not between serum MPT concentration and urinary creatinine excretion (r= 0.082, P= 0.461).

Conclusion: The concentration of serum MPT is a more reliable diagnostic parameter than that of serum creatinine as a measure of normal renal function, and renal function can be compared in subjects independently of age and muscle mass when serum MPT concentration is measured.