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

The measurement of urinary enzymes in patients with diabetes mellitus has b ecome a useful additional test for the early detection of nephropathy. Cont roversy persists concerning methods of measurement since during early stage s of insulin-dependent diabetes mellitus children develop the so-called hyp erfiltration syndrome. This study was performed to determine whether elevat ed levels of excreted creatinine influence the determination of the enzyme N-acetyl-beta-D-glucosaminidase (NAG). Reference values for NAG in single-s pot urines (units NAG/mmol creatinine) and in 24-h collections (units NAG/1 urine) were established in two different groups of 105 and 111 healthy chi ldren. 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 peri od and compared with the reference population. These results were compared with hemoglobin (Hb)A1(a-c) and fructosamine values as well as creatinine c learance of the diabetic patients. There was a direct correlation between t he NAG levels in single-spot and 24-h urine collections of diabetic patient s. However, the NAG/creatinine ratio in the single-spot urines did not corr elate with the HbA1(a-c) or fructosamine level. When 24 h collections (expr essed as NAG/l) were used, the results correlated well with HbA1(a-c) and f ructosamine. There was an inverse correlation between the creatinine cleara nce and the NAG/creatinine ratio, i.e., a high creatinine clearance correla ted with a low NAG/creatinine ratio. This was not the case for 24 h collect ions (expressed as units/l). Hence, in children with insulin-dependent diab etes mellitus 24-h urine collections should be used for urinanalysis. Param eters should not be related to creatinine, since the ratio of urinary prote in and creatinine is unreliable because of the high urinary creatinine duri ng the hyperfiltration state.