Читать книгу Fundamentals of Analytical Toxicology - Robin Whelpton - Страница 48

Different urine specimens, e.g. random, early morning, end-of-shift, 24-hour (the total urine voided over a day), may be collected in the course of metabolic or other studies. In metabolic studies, it is important to note the time of the beginning and end of the collection period so that the rate of urine production can be calculated. A random urine sample is usually a midstream specimen. A preservative, for example 2 mol L^–1 hydrochloric acid to prevent microbial growth and to stabilize phenols, may be added. Fresh urine is yellow/yellow-green in colour, but on storage in acidic solution the colour changes to yellow/brown and even to dark brown because of the oxidation of urobilinogen to urobilin. Crystals, particularly of uric acid and calcium oxalate, may form causing turbidity.

When random, early morning, or end-of-shift specimens are collected it is common practice to relate certain analytical results to a ‘fixed’ urinary constituent such as creatinine, which is considered to be excreted at a relatively constant rate in normal subjects. However, because creatinine is derived from creatine, there are situations such as muscle wasting, excessive exercise, or in bodybuilders dosing with creatine, when this is not strictly true. Note that many clinical chemistry laboratories report creatinine in mmol L^–1 (1 mmol L^–1 = 113 mg L^–1).

The concentrations of many drugs and metabolites, and of some endogenous constituents, will remain the same in acidified urine for over a week at room temperature, and for up to a month at 2–8 °C. Unacidified urine undergoes microbiological attack and many changes occur, including the complete loss of amino acids. For long term storage acidified urine can be stored at –20 °C, but it may be necessary to centrifuge the sample to remove any precipitate formed during storage prior to any analysis. Dried urine spots may also be produced in an analogous manner to dried blood spots.