title: An enzymic creatinine assay and a direct ammonia assay in coated thin films.
abstract: We developed a thin-film enzymic assay for creatinine that makes use of creatinine iminohydrolase (EC 3.5.4.21) to convert creatinine to N-methylhydantoin and ammonia. The ammonia diffuses through a semipermeable layer and is quantitated by reaction with bromphenol blue. A paired analysis of the sample on a separate coating without the enzymic reaction measures endogenous ammonia and, for samples with normal concentrations of ammonia, allows accurate determination of serum creatinine to 150 mg/L without dilution. Results of this assay (y) compare well with those by a liquid-chromatographic comparison assay (x) by linear regression (slope = 0.935, intercept = 1.13 mg/L, r2 = 0.995). It is insensitive to many substances, such as ketones and keto acids, that interfere with conventional assays. Results of the ammonia assay (y) correlate well with those by a semi-automated enzymic assay (x) based on glutamate dehydrogenase (slope = 1.068, intercept = 17.3 mumol/L, r2 = 0.985).
mesh_terms: D000641:Ammonia / Q000032:analysis*; D001978:Bromphenol Blue; D002853:Chromatography, Liquid; D003124:Colorimetry; D003404:Creatinine / Q000097:blood* / Q000378:metabolism; D005189:False Positive Reactions; D005417:Flavobacterium / Q000201:enzymology; D006801:Humans; D006827:Hydantoins; D007700:Kinetics
chemical_list: D006827:Hydantoins; C032067:1-methylhydantoin; D001978:Bromphenol Blue; D000641:Ammonia; D003404:Creatinine