What clinicians should know about urinalysis
When done properly, urinalysis is one of medicine's most helpful and cost-effective tools. It not only reveals renal abnormalities but also can detect conditions elsewhere in the body that adversely affect renal function or cause normal kidneys to excrete abnormal amounts of an end product. The importance of urinary color, odor, and
turbidity as diagnostic clues should not be overlooked. But the test's usefulness can be compromised by improper timing and collection as well as a host of other easily overlooked factors. Then there are the structural elements — RBCs, WBCs, cells from the urinary tract, and casts — that can signal everything from inflammatory vascular disease to nephritis or nothing at all but are challenging to interpret. In this article, we explore everything you should know about making urinalysis live up to its potential.
Containers must be clean, thoroughly dry, and ideally, disposable. While a spontaneously voided urine sample is generally acceptable for routine analysis, a midstream, clean-voided specimen avoids contamination with skin and urethral organisms.1 Because obtaining such a specimen is difficult in females, a catheterized sample is best when a woman's urine may require culture.
Generally, a more concentrated specimen is better, making first-morning voided urine the most useful sample.
Urine that stands at room temperature for longer than two hours is subject to bacterial overgrowth, change in pH, and dissolution of RBC and WBC casts. Urine that cannot be examined promptly should be refrigerated.
Providers can perform the test themselves or use outside laboratories that are CLIA (Clinical Laboratory Improvement Act — 1988)-certified. The CLIA waives the certification requirement for procedures determined by the FDA or CDC to have little risk of error. Microscopic examination of the urine can be done in an office-based laboratory meeting the established requirements by a physician, podiatrist, dentist, certified nurse practitioner, or physician assistant. Performance by others requires certification for procedures of moderate complexity. Quality control measures described by reagent strip manufacturers should be implemented regularly.
Color: The color of normal fresh urine ranges from pale to dark yellow to amber. Concentration, food pigments, dyes, blood, and other chemicals affect color. Urine may appear reddish-orange after rifampin or phenazopyridine (Pyridium) intake, reddish after beet ingestion, and blue-green after ingestion of methylene blue.2 A red or red-brown color could be from a food dye, ingestion of fresh beets, a drug, or the presence of porphyrins, hemoglobin, or myoglobin. Urine darkens as it stands.
Odor: The normal “urinoid” odor can be quite strong in concentrated specimens but does not imply infection, which is often associated with a pungent odor. Odor is generally not of specific diagnostic significance, although certain innate metabolic disturbances, e.g., phenylketonuria and maple syrup urine disease, may cause a characteristic odor. Fruity or sweet odors can result from diabetic ketoacidosis, and ammoniacal odors from alkaline fermentation after prolonged standing or retention in the bladder. Other odors include the sulfur smell of decomposing cystine; the fecal smell associated with a GI-bladder fistula; and the effects of drugs or diet, e.g., the characteristic odor that follows asparagus ingestion.
Turbidity: Not normally present, turbidity may be caused by excessive cellular material or protein or may develop from benign crystallization or precipitation of salts on standing (at room or refrigerator temperatures). Alkaline urine, sometimes due to infection, can appear cloudy.