Urinalysis, Microscopic, Osmolality, and pH
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
The kidney plays a key role in the excretion of by-products of cellular metabolism and regulation of water, acid-base, and electrolyte balance. Urine is produced by filtration of plasma in the renal glomeruli followed by tubular secretion and/or reabsorption of water and other compounds.
Abnormalities detected by urinalysis may reflect either urinary tract diseases (eg, infection, glomerulonephritis, loss of concentrating capacity) or extrarenal disease processes (eg, glucosuria in diabetes, proteinuria in monoclonal gammopathies, bilirubinuria in liver disease).
Screening for urinary tract diseases and some nonrenal diseases
RBCs, WBCs, renal tubular epithelial (RTE) cells, casts, squamous cells, parasites, fat, bacteria, and pathologic crystals are reported. RBCs are almost always indicative of glomerulonephritis. WBC casts are typically an indication of acute interstitial nephritis or pyelonephritis, but can also be seen in glomerulonephritides because there is often a component of accompanying interstitial nephritis. Fatty casts and free fat are often seen in patients with nephrotic syndrome or other glomerular diseases associated with significant proteinuria. Granular casts are observed in a number of disorders and are thought to be formed from partially degraded cellular casts, or are protein-derived casts. Hyaline casts are not thought to be indicative of any disease process, but increased numbers may be seen in concentrated urine specimens. Waxy casts and broad casts are most often observed in advanced renal failure. Increased numbers of RTE cells are indicators of renal tubular injury. Increased numbers of RTE cells may be caused by drugs with renal tubular toxicity (eg, cyclosporine A, aminoglycosides, cisplatin, radiocontrast media, acetaminophen overdose), interstitial nephritis, hypotension (surgical, sepsis, obstetric complications), and heme pigments from hemoglobinuria or myoglobinuria from rhabdomyolysis (eg, alcoholism, heat stroke, seizures, sickle cell trait). Newborns often shed RTE cells in their urine.
Based on careful review of all available published outcome studies with results of detailed hematuria workups within actual patient populations, a panel from the American Urological Association recommends that patients with >3 RBCs per high-power field (hpf) in 2 out of 3 properly collected urine specimens should be considered to have microhematuria, and hence evaluated for possible pathologic causes. However, the panel also noted that there is no absolute lower limit for hematuria, and risk factors for significant disease should be taken into consideration before deciding to defer an evaluation in patients with only 1 or 2 RBCs per hpf. High-risk patients, especially those with a history of smoking or chemical exposure, should still be considered for a full urologic evaluation even after 1 properly performed urinalysis documented the presence of at least 3 RBCs per hpf. In certain patients, even 1 or 2 RBCs per hpf might merit evaluation. The presence of squamous cells suggests that the sample may not have been an optimal, clean-catch specimen and could be contaminated with skin flora.
Osmolality is an index of the solute concentration of osmotically active particles, principally sodium, chloride, potassium, and urea. Glucose can contribute significantly to the osmolality when present in substantial amounts. The ability of the kidney to maintain both tonicity and water balance of the extracellular fluid can be evaluated by measuring the osmolality of the urine. More information concerning the state of renal water handling or abnormalities of urine dilution or concentration can be obtained if urinary osmolality is compared to serum osmolality. Normally, the ratio of urine osmolality to serum osmolality is 1.0 to 3.0, reflecting a wide range of urine osmolality. The reference ranges are as follows:
0 to 12 months: 50 to 750 mOsm/kg
>12 months: 150 to 1,150 mOsm/kg
Please note above the age of 20 years there is an age-dependent decline in the upper reference range of approximately 5 mOsm/kg/year.
Urine pH is affected by diet, medications, systemic acid-base disturbances, and renal tubular function. pH may affect urinary stone formation. For example, urine pH <6.0 may help reduce the tendency for calcium phosphate stones and pH >6.0 may reduce the tendency for uric acid stone formation.
Ketones are produced during metabolism of fat. Increased ketones may occur during physiological stress conditions such as fasting, pregnancy, strenuous exercise, and frequent vomiting. Ketones may appear in the urine in large amounts, before serum ketone is elevated, under the following conditions:
-Diabetic individuals who are unable to efficiently utilize glucose due to a lack of insulin
-Individuals with other abnormalities of carbohydrate or lipid metabolism
Bilirubinuria is an indicator of liver disease and biliary tract obstruction.
Hemoglobinuria is an indicator of intravascular hemolysis. The test is equally sensitive to myoglobin as to hemoglobin (Hgb). The presence of Hgb, in the absence of RBCs, is consistent with intravascular hemolysis. RBCs may be missed if lysis occurred prior to analysis; the absence of RBCs should be confirmed by examining a fresh specimen. The presence of myoglobin may be confirmed by MYOU / Myoglobin, Urine.
Urine can contain a variety of reducing substances (sugars [glucose, galactose, sucrose, fructose, lactose, maltose], ascorbic acid, drugs, etc), compounds so termed because of their ability to reduce cupric ions. The primary reducing substances of medical significance are the sugars, glucose (diabetes) and galactose (galactosemia). Other sugars may be found but are not of clinical significance. Because glucose also is detected by glucose-specific dipstick reagents, the test for reducing substances is performed to detect galactose.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Elevated specific gravity, elevated protein, and large amounts of ascorbic acid may cause false-negative results. Oxidizing substances such as hypochlorite and chlorine may cause false-positive results. The test is equally sensitive to hemoglobin (Hgb) and to myoglobin. The presence of Hgb, in the absence of RBCs, is consistent with intravascular hemolysis. RBCs may be missed if lysis occurred prior to analysis; the absence of RBCs should be confirmed by examining a fresh specimen. The presence of myoglobin may be confirmed by MYOU / Myoglobin, Urine.
This test reacts with sufficient quantities of any reducing substance in the urine; it is not specific for glucose. Urine specimens with low specific gravity that contain glucose may give slightly-elevated results. Metabolites of some sulfa drugs and methapyrilene compounds may interfere with the sensitivity of the test. X-ray contrast media in urine produces reduced and false-negative glucose results.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
Clinical References Provides recommendations for further in-depth reading of a clinical nature
Grossfeld GD, Litwin MS, Wolf JS, et al: Evaluation of asymptomatic microscopic hematuria in adults: the American Urological Association best practice policy-part I: definition, detection, prevalence, and etiology. Urology 2001;57:599-603