Understanding Urinalysis

Clues for the Obstetrician-Gynecologist

Sarah Coad; Boris Friedman; Roxana Geoffrion

Disclosures

Expert Rev of Obstet Gynecol. 2012;7(3):269-279. 

In This Article

Urinalysis General Characteristics

Color

Urine color is mainly determined by chemical content and concentration. Urine is usually yellow due to the naturally occurring pigment urobilin, formed as a result of heme breakdown.[11]

A change in urine color may be due to benign or pathological causes. Red urine may suggest ingestion of beets, blackberries, rhubarb, phenolphthalein or rifampin; on the other hand, it can also suggest hematuria, myoglobinuria or porphyria. Orange urine may be due to phenothiazines or phenazopyridine or may represent bile pigments in cases of bile duct obstruction. Dark yellow urine may be related to carrots, cascara or riboflavin (vitamin B2), as well as to dehydration. Greenish-blue urine may be found in patients taking amitriptyline, cimetidine, triamterene, promethazine or after sedation with propofol,[12] or from dyes such as indigo carmine and methylene blue; it is also indicative of pseudomonal infection or the presence of biliverdin, an oxidation product of bilirubin, which may be seen in the urine of patients with elevated serum bilirubin levels.[13] Brown urine may be a consequence of ingestion of cascara, senna, levodopa, metronidazole, nitrofurantoin or some antimalarial agents. Bile pigments, myoglobin, methemoglobin and melanin may also color urine dark brown. The presence of melanin in urine may be associated with metastatic melanoma.[14] Cloudy urine may be due to a diet high in purine-rich foods such as meat and beans, or may signal phosphaturia, pyuria, chyluria, lipiduria and hyperoxaluria.[11,15]

In summary, urine color varies between individuals and is dependent on hydration state, diet, medications and pathology.

Urine-specific Gravity

Determination of USG is a convenient and rapidly obtained indicator of urine osmolality.[16] Osmolality is a measure of concentration and is defined as the number of particles dissolved in a solution. In patients with normal renal function, this is primarily determined by antidiuretic hormone. By contrast, USG is a measure of the weight of the urine compared with that of an equal volume of distilled water.[17] The assumption made with this technique is a linear relationship between USG and osmolality. Most reagent strips contain a polyionic polymer with binding sites saturated with hydrogen ions. The release of these hydrogen ions when competitively replaced with urinary cations (e.g., Na+ and K+) causes a change in the pH-sensitive indicator dye. USG values measured by dipstick tend to be falsely high at a urine pH of less than six and falsely low if the pH is greater than 7.[18] It is important to remember that the relationship between USG and osmolality is altered when there are significant quantities of albumin, glucose and urea present in the urine. In these situations, the USG may be elevated, indicating highly concentrated urine when the urine osmolality is in the normal range.

Clinically, USG reflects the hydration status of a patient and the concentrating ability of the kidneys. Normal USG can range from 1.003 to 1.030. A value <1.010 indicates relative hydration and a value >1.020 indicates relative dehydration.

Increased USG is associated with glycosuria and syndrome of inappropriate antidiuretic hormone hypersecretion. Decreased USG is associated with diuretic use, diabetes insipidus, adrenal insufficiency, aldosteronism and impaired renal function. Self-monitoring of USG may be useful for stone-forming patients, as keeping USG low indicates adequate hydration and may decrease recurrence.[19]

In summary, in most cases, USG correlates with osmolality and is related to patient hydration status.

Urine pH

Urine pH ranges from 4.5 to 8 but is normally slightly acidic (i.e., 5.5 to 6.5). Dipsticks use double indicators (methyl red and bromothymol blue) to give a broad range of colors at different pH values. Urinary pH can reflect serum pH, although larger fluctuations are noted in urine.[16]

Alkaline urine (pH >7.0) may be suggestive of infection with urea-splitting organisms (e.g., Proteus mirabilis).[16] Urease-producing bacteria convert ammonia to ammonium ions, elevating urine pH. Vegetarian diet, diuretic therapy, vomiting, gastric suction and alkali therapy can also cause a high urine pH.[2] In both types I and II renal tubular acidosis, the serum is acidemic while the urine is alkalotic, due to the continued loss of bicarbonate in the urine.

Acidic urine (pH <5.0) is seen most commonly in conditions associated with respiratory and metabolic acidosis. Low urine pH is also associated with the ingestion of large amounts of meat and uric acid calculi.[2,15]

In summary, urine pH varies between individuals and is dependent on serum pH, diet, medications and pathology.

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