Specific gravity Information and Courses from MediaLab, Inc.

Normal cerebrospinal fluid has the following characteristics:colorlessclearno clot presentspecific gravity of 1.006 - 1.008pH 7.3When the specimen is received in the laboratory, the macroscopic examination is performed immediately. The specific gravity examination may be optional in some laboratories.

False negative results occur when elements present in the urine interfere with either the enzymatic reaction or prevent the oxidation of potassium iodide. Examples of such substances include: large quantities of ketones aspirin ascorbic acid > 50 mg/dL with some reagent strips levadopa 5-hydroxyindoleacetic acid homogentisic acid sodium fluoride ( a preservative)A specific gravity higher than 1.020 may lower glucose reagent sensitivity, especially in the presence of a high urine pH. Exposing reagent strips to excess humidity may also reduce glucose reagent reactivity.Check the package insert of the reagent strips used in your laboratory for interfering substances that may affect glucose results.

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.

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.

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.

Measurement of specific gravity provides information regarding a patient's state of hydration or dehydration. It also can be used to determine loss of renal tubular concentrating ability.

The microscopic examination was traditionally performed on all urine specimens after macroscopic exam, specific gravity and chemical tests were completed. Today, many laboratories perform a urine microscopic only if preliminary evaluation indicates the need for microscopic examination. Such laboratories must have criteria determining the specimens on which a urine microscopic will be determined. The microscopic exam is often important in detecting and evaluating renal and urinary tract disorders as well as other systemic diseases.

The results are abnormal. The presence of glucose is not a normal finding. However, the two glucose methods correlate well with each other.The specific gravity does not correlate well with the glucose. A large amount of glucose should elevate the urine specific gravity. The specific gravity result should therefore be rechecked before reporting. The presence of 3+ bacteria, does not correlate well with scant white cells and lack of turbidity. The technologist should question whether the specimen was held at room temperature for a protracted period prior to examination.