The hydroxyproline index is a biochemical test of protein nutritional status in children, based on measuring hydroxyproline — an amino acid abundant in collagen — excreted in the urine. Because the body’s collagen turnover reflects growth and protein adequacy, low urinary hydroxyproline (expressed relative to creatinine) signals protein malnutrition and stunted growth. Developed for field surveys where weighing and measuring children is impractical, the index gave nutrition workers a laboratory window onto protein status from a simple urine sample. Modern assessment prefers anthropometry and blood markers, but the hydroxyproline index remains historically important — one of the first attempts to quantify malnutrition biochemically rather than clinically.