Urinary N-acetyl-beta-D-glucosaminidase (NAG) is measured in a hospitalised patient receiving aminoglycoside therapy whose serum creatinine has just begun to rise. NAG is useful in this setting because:

• A It is filtered freely at the glomerulus and reflects GFR decline earlier than creatinine
• B It inhibits aminoglycoside uptake and predicts drug toxicity before exposure
• C It is produced by damaged glomerular epithelial cells and detects early glomerulosclerosis
• D It is a lysosomal brush border enzyme of proximal tubular cells released when tubules are injured ✓

Explanation

NAG is a large lysosomal enzyme resident in the brush border of proximal convoluted tubular cells. Because of its molecular size it cannot pass through an intact glomerulus, so any urinary NAG must originate from injured tubular cells, making it a sensitive early marker of tubular damage such as aminoglycoside nephrotoxicity or acute tubular necrosis. Option A describes cystatin C, which assesses GFR, not tubular integrity, and option C confuses glomerular with tubular injury.

Reference: Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.