Urines Information and Courses from MediaLab, Inc.
These are the MediaLab courses that cover Urines and links to relevant pages within the course.
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| Basis of the Urine pH Test Dipsticks measure pH using methyl red and bromthymol blue indicator dyes. The color change that occurs in this test area correlates with the urine's pH. Sensitivity to pH ranges from 5.0 (acid pH) to 9.0 (alkaline pH) on a urine reagent strip. | View Page |
| Test Sensitivity This test is sensitive to 0.06-0.1 mg/dL nitrite ion in urines with a low specific gravity and ascorbic acid concentrations of less than 25 mg/dL. Pink spots or pink edges should not be interpreted as a positive result because some medications can color urine red or turn red in an acid environment. Any degree of uniform pink color should be considered positive, suggesting the presence of 105 organisms/mL. Detection of low levels of nitrite ion may be enhanced by comparing the activated test strip to a white background. It is important to note that color development is NOT proportional to the number of bacteria present. The test is specific for nitrites and does not react with any other substances normally present in urine. Negative results do not necessarily rule out a urinary tract infection because yeasts or gram-positive bacteria unable to reduce nitrites may be the causative agent. | View Page |
| False Negative False negative results may occur in the presence of significant levels of protein or glucose and in urines with high specific gravity which may crenate the white blood cells causing them to be come unable to release esterases. Some drugs such as Cephalexin (Kelfex®), Cephalothin Keflin®) or high concentrations of oxalic acid may also cause decreased test results. Tetracycline may cause decreased activity, and high levels of the drug may cause a false negative reaction. Large amounts of ascorbate may cause false negative results. | View Page |
| Measuring Specific Gravity The reagent strip measures specific gravity in increments of 0.005 with readings from 1.000 to 1.035. The test principle is based on a change in pKa (the negative log of the acid disassociation) of certain pretreated electrolytes (methylvinyl ether/maleic anhydride) in relation to ionic concentration of the urine. These electrolytes in the reagent area contain acid groups which disassociate according to the ionic concentration of the specimen. The more ions in the specimen, the more acid groups will become disassociated, releasing hydrogen ions and causing a more acid pH. The reagent area contains a pH indicator (bromthymol blue) which demonstrates the change in pH. The higher the specific gravity of the urine specimen, the more acidic the reagent area will become. The colors of the reagent area will range from deep blue-green in urines of low ionic concentration to green-to-yellow green in urines of increasing ionic concentration, and consequently, higher specific gravity. | View Page |
| Advantages and Limitations of the Reagent Strip Method for Specific Gravity Specific gravity measured with the reagent strip method correlates well with gravimetric measurement, and, unlike the gravimetric or refractometer methods, does not need to be corrected for glucose or protein. Cloudy/turbid urines do not need to be clarified before measuring specific gravity with the reagent strip method. It is the recommended method for determining specific gravity if a urine specimen contains x-ray contrast media or plasma expanders. Alkaline urine can affect the indicator system and lower the specific gravity result on the reagent pad. If the result is being read visually, it is recommended that .005 be added to the result when the pH is alkaline. Most dipstick readers, however, will automatically adjust the specific gravity reading for pH. A specific gravity reading higher than the reagent strip range would need to be measured by another method, and may require dilution. | View Page |