ELECTROLYTE MEASUREMENTS: SODIUM (Na), POTASSIUM (K), AND CHLORIDE (Cl)
The concentrations of the major electrolytes in venous plasma or serum can be determined using ion-selective electrodes for each one. The major ones are sodium (Na), potassium (K), and chloride (Cl). A major preanalytical error involves aggressive handling of the tube (shaking instead of gentle inversion) prior to analysis. It is frequently manifested by an artifactually high value for potassium. Ionized or free calcium, not bound to protein, can also be measured using an electrode specific for calcium.
ASSAYS MEASURING CONCENTRATION OF COMPOUNDS BY SPECTROPHOTOMETRY
There are many assays involving spectrophotometry as the detection method for quantitation of compounds in the clinical laboratory. Changes in light absorption at a selected wavelength are measured for the specific reagents introduced into the chemical reactions, often linked reactions, measurable by spectrophotometry. In a typical assay, the patient sample is pipetted into a cuvette, and then reagents are added to initiate and maintain chemical reactions until a colored product is generated. The extent of the change in light absorbance detected by spectrophotometry is proportional to the amount of the compound of interest which is present.
A whole blood specimen collected from an artery is injected into a blood gas analyzer which can determine pH, pO2, and pCO2. There are electrodes specific to hydrogen for pH, carbon dioxide for pCO2, and oxygen for pO2 in the blood gas instrument. These parameters are determined specifically for arterial blood, as the values in venous blood are markedly different.
The first step in performing an analysis of urine is gross inspection of the urine for color and turbidity changes. A variety of colors other than yellow can be associated with a urine specimen, and samples from patients with a variety of conditions may be highly turbid. The chemical analysis of different compounds in the urine involves the use of a reagent pad that can detect many different compounds semiquantitatively. These include specific gravity, pH, leukocytes, nitrite, protein, glucose, ketones, urobilinogen, bilirubin, and blood. Some strips contain fewer pads than others. The strip is immersed in the urine for about one second, and the color changes of the reagent pads are noted at the appropriate time after removal of the strip from the urine. The semiquantitative measurement for each of the parameters can be read by spectrophotometry or viewed with the naked eye. In that circumstance, the results for each pad on the strip are established by comparison to pad colors associated with the different amounts of what ...