Influence of hyperfiltration on the measurement of urinary N-acetyl-beta-D-glucosaminidase

The measurement of urinary enzymes in patients with diabetes mellitus has become a useful additional test for the early detection of nephropathy. Controversy persists concerning methods of measurement since during early stages of insulin-dependent diabetes mellitus children develop the so-called hyperfiltration syndrome. This study was performed to determine whether elevated levels of excreted creatinine influence the determination of the enzyme N-acetyl-beta-D-glucosaminidase (NAG). Reference values for NAG in single-spot urines (units NAG/mmol creatinine) and in 24-h collections (units NAG/l urine) were established in two different groups of 105 and 111 healthy children. NAG was then measured in single-spot urines (as NAG/mmol creatinine) and in collection urines of 30 diabetic children within the same 24-h period and compared with the reference population. These results were compared with hemoglobin (Hb)A1(a-c) and fructosamine values as well as creatinine clearance of the diabetic patients. There was a direct correlation between the NAG levels in single-spot and 24-h urine collections of diabetic patients. However, the NAG/creatinine ratio in the single-spot urines did not correlate with the HbA1(a-c) or fructosamine level. When 24 h collections (expressed as NAG/l) were used, the results correlated well with HbA1(a-c) and fructosamine. There was an inverse correlation between the creatinine clearance and the NAG/creatinine ratio, i.e., a high creatinine clearance correlated with a low NAG/creatinine ratio. This was not the case for 24 h collections (expressed as units/l). Hence, in children with insulin-dependent diabetes mellitus 24-h urine collections should be used for urinanalysis. Parameters should not be related to creatinine, since the ratio of urinary protein and creatinine is unreliable because of the high urinary creatinine during the hyperfiltration state.