Simple Metabolic Screening Tests

Abstract

This chapter provides an overview of selected colorimetric “test-tube” assays employed in the Clinical Biochemical Genetics laboratory. The cyanide nitroprusside test determines the presence of free sulfhydryl or disulfide compounds in urine samples. The test is useful for the rapid estimation of cysteine (cystine) associated with heritable cystinuria. The Ehrlich’s test is employed to identify the presence of porphobilinogen or urobilinogen in urine, with applicability to the inherited porphyrias. The dinitrophenylhydrazine assay detects α-keto acids in urine, and assists in the detection of phenylketonuria, maple-syrup urine disease, and disorders of tyrosine metabolism. The nitrosonaphthol test is also useful in identifying urine tyrosine metabolites, although a common confound is transient tyrosinemia of the newborn. The use of MULTISTIXR reagent strips, with the capacity to qualitatively assess a number of urine intermediates and parameters, can provide evidence for a number of metabolic disorders and insight into renal function. Despite their qualitative nature, these simple assays are still useful in the laboratory, provided the nutritional and medication status of the patient is documented.