Definition
- The dipstick reagent strip method is commonly used to assess the chemical evaluation of urine. The most frequently performed chemical tests using reagent test strips are specific gravity, pH, protein, glucose, ketones, blood, leukocyte esterase, nitrite, bilirubin, and urobilinogen.
- Specific gravity: Specific gravity is a measure of the dissolved substances present in the urine. It is a physical property of urine and an expression of concentration.
- Color: The color of the specimen is measured by comparison to four known wavelengths of light (red, violet, blue, and green), which are used to determine the color and hue of the sample.
- Clarity: The clarity or turbidity of the urine specimen is measured by passing a light beam through the sample and measuring the scattered light. The amount of scattered light increases as the specimen becomes more turbid. Clarity is reported as clear, turbid, or extremely turbid.
- pH: Along with the lungs, the kidneys are the major regulator of acid " base balance. pH testing provides valuable information for assessing and managing disease and determines the suitability of a specimen for chemical testing. Freshly voided urine has a pH of 5.0 " 6.0. The pH of urine can be controlled by dietary regulation and medication.
- Glucose: Glucosuria is usually indicative of hyperglycemia due to diabetes but can also be seen in patients with other causes for hyperglycemia, in patients with renal tubular dysfunction, and in pregnancy due to increased glomerular filtration. In children especially younger than 2 years of age, it is important to perform a screening test for reducing sugar.
- Protein: The presence of protein in urine is mostly indicative of renal disease, but its appearance in the urine does not always signify renal disease. The strip is primarily sensitive to albumin.
- Bilirubin: The appearance of urinary bilirubin can be a sign of liver disease or extrahepatic or intrahepatic biliary obstruction.
- Urobilinogen: The normal urine has a small amount of urobilinogen. Increased amounts appear in hemolytic anemias and liver dysfunction.
- Blood: Equally specific for RBCs, Hb, or myoglobin present in the urine. Hematuria can be seen due to bleeding as a result of trauma or irritation. Hemoglobinuria occurs when there is lysis of RBCs in the urinary tract, intravascular hemolysis, or transfusion reactions. Very dilute or extremely alkaline urine can also lyse the cells. Myoglobinuria indicates muscular destruction that may appear in hypothermia, convulsions, and extensive exertions.
- Ketones: Ketonuria appears when there is an increased use of fat instead of carbohydrate for metabolism. Conditions of ketonuria include DM, vomiting, and inadequate intake of carbohydrates due to starvation or weight reduction, or pregnancy.
- Nitrite: Bacteria, specifically gram-negative bacteria, are detected. This analysis provides a rapid and economical means of detecting significant bacteriuria caused by nitrate reducing bacteria. It is limited by various factors, including characteristics of microorganisms, dietary factors, urinary retention time, and specimen storage.
- Leukocytes: The presence of WBCs is an indicator of inflammation; lysed and intact WBCs are detected.
- Normal range: see Table 16.81.
TABLE 16 " 81Reference Values for UrinalysisView LargeTABLE 16 " 81Reference Values for Urinalysis Test Reference Range Color Yellow Appearance Clear Specific gravity 1.005 " 1.030 pH 4.6 " 8.0 Protein Negative Glucose Negative Ketone Negative Bilirubin Negative Occult blood Negative Nitrite Negative Urobilinogen Normal Leukocyte esterase Negative White blood cells 0 " 2/HPF Red blood cells 0 " 2/HPF Hyaline casts 0 " 2/LPF Bacteria None
Use
- Frequently performed screening test for metabolic and kidney disorders and for UTIs.
Interpretation
- For specific causes of increased and decreased values of constituents, see individual tests.
Limitations
See Table 16.82.
TABLE 16 " 82Interferences Which May Cause False-Positive or False-Negative Test ResultsView LargeTABLE 16 " 82Interferences Which May Cause False-Positive or False-Negative Test Results Analyte Causes of False-Positive Results Causes of False-Negative Results Specific gravity High protein concentrations between 100 and 500 mg/dL and presence of keto acids >1 g/dL glucose and urea concentrations pH No interferences known Blood Menstrual contamination, microbial peroxidases, strong oxidizing agents (soap and detergents) Ascorbic acid, high specific gravity, captopril Dehydration, exercise Leukocyte esterase Highly colored substances mask results, beets, drugs (phenazopyridine), vaginal contamination of urine High specific gravity, increased glucose, protein, strong oxidizing agents, drugs such as gentamicin, cephalosporins, presence of lymphocytes Nitrite Highly colored substances mask results, beet ingestion, drugs (phenazopyridine), improper storage with bacterial proliferation, exposure of dipstick to air Ascorbic acid, various factors that inhibit nitrite formation despite bacteriuria Protein Alkaline urine, alkaline drugs, improperly preserved specimen, contamination with quaternary ammonium compounds; highly colored substances mask results, beet ingestion, drugs (phenazopyridine) Presence of protein other than albumin Glucose Strong oxidizing agents such as bleach, peroxidase contaminants Ascorbic acid, improperly stored specimens (glycolysis) Ketones Compounds containing free sulfhydryl groups such as captopril, N-acetylcysteine, highly pigmented urine, atypical colors with phenylketones and phthaleins, large amounts of levodopa metabolites, acidic urine, elevated specific gravity Improper storage, resulting in volatilization, bacterial breakdown Bilirubin Drug-induced color changes such as phenazopyridine, indicant-indoxyl sulfate, large amounts of chlorpromazine metabolites Ascorbic acid, high nitrite concentrations, improper storage resulting in oxidation or hydrolysis to nonreactive biliverdin and free bilirubin, light exposure, chlorpromazine (Thorazine), selenium Urobilinogen Atypical colors caused by sulfonamides, p-aminobenzoic acid, p-aminosalicylic acid, substances that induce color mask results, beet ingestion, elevated nitrite levels Formalin, improper storage resulting in oxidation to urobilin
Suggested Reading
1Brunzel NA. Fundamentals of Urine and Body Fluid Analyses, 2nd ed. Philadelphia, PA: Saunders; 2004.