To the Editor.
—Sherman et al1 reported finding an increased urinary excretory level of N-acetyl-β-glucosaminidase (NAG) and β2-microglobulin (β2M) in various renal diseases. They also compared the excretory levels of NAG and β2M in various patient subgroups of glomerulonephritis. I have done a similar study with similar results (unpublished data, February 1982). I also agree with Sherman et al that in general NAG excretion in progressive disorders is higher than in the stable lesions.However, before comparing these subgroups one must consider the glomerular filtration rate (GFR). There is a verifiable negative correlation between the GFR and the NAG excretion per gram of serum creatinine.2 This interrelationship precludes direct comparison of NAG excretion in different patient groups, since the GFR often varies considerably. Kunin et al3 has pointed out that the high NAG-serum creatinine ratio in renal failure is due