Nitroanaline detects massive elevations of methylmalonic acid. Ferric chloride is a screening test for phenylketones. Nitroprusside screens for massive elevations of cystine and homocysteine. Additional tests often used to evaluate potential inborn errors include urine organic acids and urine and plasma amino acids. Common reasons for false negative results include collection of dilute urine, intermittent excretion of abnormal metabolites, collection after the start of supportive therapy, and the presence of protein, glucose, ketones, or blood.
Drug therapy is a common reason for false positive results.
Testing for inborn errors of metabolism in the acutely ill newborn.