Specimen Collection and Storage

In individuals with a normal hematocrit, fasting whole-blood glucose concentration is approximately 10% to 12% lower than plasma glucose. Although the glucose concentra- 

Cerebrospinal fluid (CSF) may be contaminated with bacteria or other cells and should be analyzed for glucose immediately. If a delay in measurement is unavoidable, the sample should be centrifuged and stored at 4 C or -20 C. 

In 24-hour collections of urine, glucose maybe preserved by adding 5 mL of glacial acetic acid to the container before starting the collection. The final pH of the urine is usually between 4 and 5, which inhibits bacterial activity. Other preservatives that have been proposed include 5 g of sodium benzoate per 24-hour specimen or chlorhexidine and 0.1% sodium nitrate (NaNs) with 0.01% benzethonium chloride. These may be inadequate, and urine should be stored at 4 °C during collection. Urine samples may lose as much as 40% of their glucose after 24 hours at room temperature.  

Hexokinase and glucose oxidase are the two main types of methods used to measure glucose in body fluids. Other methods are found on the Evolve site that accompanies this book. 

Hexokinase procedures in which indicator reactions produce colored products are also available, enabling absorbance to be measured in the visible range. An oxidation-reduction system containing phenazine methosulfate (PMS) and a substituted tetrazolium compound, 2-(p-iodophenyl)-3-p-nitrophenyl-5-phenyltetrazolium chloride (INT), is reacted with NADPH formed in the reaction. The reduced INT is colored with maximum absorbance at 520 nm. The PMS-INT color developer must be refrigerated 

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Specimen Collection and Storage. The preferred specimen is serum. Specimens may be stored at 4 °C for 1 week or at -20 °C for 30 days. Repeated freezing and thawing, mixing using a vortex mixer, or vigorous agitation of serum specimens should be avoided, because such treatment causes denaturation of proteins. The use of hemolyzed or hpemic specimens is also not recommended. 

In the following, sources of error that are important in the process of specimen collection and storage, as well as the physiological factors that cause error in the interpretation of the analysis of trace elements in human tissues are treated. 

VARIATION ASSOCIATED WITH SPECIMEN COLLECTION AND STORAGE... 

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