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Simple Tests K. Michael Gibson and Marinus Duran

A variety of rapid qualitative tests (colorimetric, dipstick, precipitate, color and smell, etc.) are useful in both specialist and nonspecialist laboratories to assist in the differential diagnosis of inherited metabolic disorders. Most are limited by some level of interference, yet these tests still have important utility, especially in emergency situations. The color and odor of a patient’s urine may be a valuable analysis for initial testing (Tables 48.1 and 48.2). Odor can only be reliably interpreted when the urine is preserved in an adequate way (pH 5–7, no signs of bacterial contamination as evidenced by a negative nitrite dipstick). Alkaptonuria patients show a rapid blackening of the urine upon standing; this process can be speeded up by adding a few drops of an ammonia solution to the urine test tube. The ferric chloride test (Table 48.3) is employed to look for the presence of oxoacids (formed in transamination or oxidation-reduction reactions). This test has been routinely used in the identification of classical phenylketonuria, but several other species (in addition to the intermediates of phenylalanine metabolism) react with ferric chloride to form a number of colored complexes. An alternative for the ferric chloride test is the Phenistix dipstick. Reducing substances in urine (see Table 48.4; also commonly referred to the ClinitestR, Bayer Corporation) reacts with a broad spectrum of reducing sugars in urine with the formation of colored complexes (green to orange). It is commonly used for the detection of urine galactose on the suspicion of galactosemia in neonates with severe liver

K.M. Gibson (*) Section of Clinical Pharmacology, Washington State University, SAC 525M, 412 E. Spokane Falls Blvd, Spokane, WA 99202-2131, USA e-mail: [email protected] M. Duran Laboratory Genetic Metabolic Diseases, Academic Medical Center, F0-224 Meibergdreef 9, Amsterdam 1105AZ, The Netherlands e-mail: [email protected]

disease and the renal Fanconi syndrome. One should be aware that the Fanconi syndrome includes renal glucosuria; hence the presence of galactose cannot be deduced from the positive Clinitest. Dinitrophenylhydrazine (DNPH) reacts with α-keto acids to produce insoluble hydrazones, forming precipitates in urine samples (Table 48.5). Conversely, Acetest analysis (urine) complexes with urinary ketones (Bayer Corporation) (Table 48.5). Parallel use of DNPH and Acetest provides slightly more diagnostic capacity. A positive result of either test will always be followed up by an immediate analysis of urine organic acids. Acetest is also frequently used for the home monitoring of patients with propionic and methylmalonic acidurias; it will give a good indication for the catabolic state of the patient, necessitating dietary intervention. This test will be of use in establishing hypoketotic hypoglycemia although the degree of ketonuria in several fatty acid oxidation disorders may be marked, especially MCAD. The cyanide nitroprusside test (or Brand reaction)