PROVIDER-PERFORMED MICROSCOPY PROCEDURES
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Urinalysis (Urine Dipstick and Sediment Examination)
Collection and Preparation of Specimen
- Obtain a midstream, clean-catch urine specimen in a clean container.
- Examine the specimen while fresh (within 2 hours; do not refrigerate specimen); otherwise, bacteria may proliferate, casts and crystals may dissolve, and particulate matter may settle out.
- Place 10 mL in a conical test tube and centrifuge at 2000–3000 rpm for 5 minutes. Do not apply brake at the end of centrifugation to avoid re-suspension of sediment.
- Invert the tube and drain off the supernatant without dislodging the sediment button. Return the tube to an upright position, and re-suspend the sediment by gently tapping the bottom of the tube.
- Place a drop of sediment on a glass slide, cover it with a coverslip, and examine under the microscope; no stain is needed.
- While the urine is being centrifuged, examine the remainder of the specimen by inspection and reagent strip (dipstick) testing.
- Inspect the specimen for color and clarity. Normally, urine is light yellow (due to urochrome). Intense yellow urine is caused by urine concentration (dehydration) or B vitamin supplements; dark orange urine, by ingestion of the urinary tract analgesic phenazopyridine; orange-reddish urine, by rifampin or rifabutin therapy; red urine, by erythrocytes, hemoglobinuria, myoglobinuria, porphyrins, senna, or beets; green urine, by Pseudomonas infection or iodochlorhydroxyquin or amitriptyline therapy; brown urine, by bilirubinuria or fecal contamination; black urine, by intravascular hemolysis, alkaptonuria, melanoma, or methyldopa therapy; and milky white urine, by pus, chyluria, or amorphous crystals (urates or phosphates). Turbidity of urine is caused by pus, red blood cells, or crystals.
- Reagent strips provide information about specific gravity, pH, protein, glucose, ketones, bilirubin, blood (heme), nitrite, and leukocyte esterase (Table 2–3). Dip a reagent strip in the urine and compare it with the interpretation guide chart on the bottle. Follow the timing instructions carefully. Note: Reagent strips cannot be relied on to detect some proteins (eg, globulins, light chains) or reducing sugars (other than glucose). Falsely positive protein results may be obtained with alkaline urine (eg, urine pH > 8.0); sulfosalicylic acid (SSA) test can be used to confirm the presence of protein. Positive bilirubin on a reagent strip should be confirmed by an Ictotest tablet. Substances that cause abnormal urine color may affect the readability of test pads on reagent strips (eg, visible levels of blood or bilirubinuria and drugs containing dyes, nitrofurantoin, rifampin or rifabutin).
- Record and report the results.
Manual Microscopic Urine Sediment Examination
- Examine the area under the coverslip under low-power (10×) and high-dry (40×) lenses for cells, casts, crystals, and bacteria.
- Cells may be red cells, white cells, squamous cells, transitional (bladder) or tubular epithelial cells, or atypical (tumor) cells. Red cells suggest upper or lower urinary tract infections (cystitis, prostatitis, pyelonephritis), glomerulonephritis, collagen vascular disease, trauma, renal calculi, tumors, drug reactions, and structural abnormalities (polycystic kidneys). White cells suggest inflammatory processes such as urinary tract infection (most common), collagen vascular disease (eg, lupus), or interstitial nephritis. Red cell casts are considered pathognomonic of glomerulonephritis; white cell casts, of pyelonephritis; and fatty (lipid) casts, of nephrotic syndrome.
- Presence of crystals is often of no clinical significance. Presence of casts, however, is associated with various pathologic conditions. For example, WBC (leukocyte) casts are seen in patients with pyelonephritis and interstitial nephritis; RBC (erythrocyte) casts in acute glomerulonephritis, lupus nephritis, Goodpasture syndrome, and subacute bacterial endocarditis; renal epithelial casts in toxic tubular necrosis; waxy casts in severe chronic renal disease and amyloidosis; and fatty casts in nephrotic syndrome and diabetes mellitus.
|Specific gravity||1.001–1.035||1.000–1.030b||Highly buffered alkaline urine may cause low specific gravity readings. Moderate proteinuria (100–750 mg/dL) may cause high readings. Loss of concentrating or diluting capacity indicates renal dysfunction. If the specific gravity of a random urine specimen is ≥ 1.023, the concentrating ability of the kidneys can be considered normal.|
|pH||4.6–8.0||5.0–8.5 (visually)b||Excessive urine on strip may cause protein reagent to run over onto pH area, yielding falsely low pH reading. Bacterial growth by certain organisms (eg, Proteus ) in a specimen may cause a marked alkaline shift (urine pH > 8), usually because of urea conversion to ammonia.|
|Protein||Negative <15 mg/dL||15–30 mg/dL albumin|| Test is based on protein-error-of-indicators principle. False-positive readings can be caused by highly buffered alkaline urine. Reagent is more sensitive to albumin than other proteins. A negative result does not rule out the presence of globulins, hemoglobin, light chains (Bence Jones) proteins, or mucoprotein. |
1+ = 30 mg/dL 3+ = 300 mg/dL
2+ = 100 mg/dL 4+ = ≥ 2000 mg/dL
|Glucose||Negative (<15 mg/dL or <50 mg/day)||75–125 mg/dL|| Test is based on a double sequential enzyme reaction (glucose oxidase and peroxidase), and is specific for glucose. False-negative results occur with urinary ascorbic acid concentrations ≥ 30 mg/dL and with ketone body levels ≥ 40 mg/dL. Test reagent reactivity also varies with specific gravity and temperature. |
Trace = 100 mg/dL 1 = 1000 mg/dL
¼ = 250 mg/dL 2 = ≥ 2000 mg/dL
½ = 500 mg/dL
|Ketone||Negative||5–10 mg/dL acetoacetate|| Test does not react with acetone or β-hydroxybutyric acid. False-positive (trace) results may occur with highly pigmented urines or those containing levodopa metabolites or sulfhydryl-containing compounds (eg, Mesna). Trace results may occur during physiological stress conditions (fasting, pregnancy, strenuous exercise). |
Trace = 5 mg/dL Moderate = 40 mg/dL
Small = 15 mg/dL Large = 80–160 mg/dL
|Bilirubin||Negative (≤ 0.02 mg/dL)||0.4–0.8 mg/dL|| Positive (conjugated) bilirubin indicates hepatitis. False-negative readings can be caused by ascorbic acid concentrations ≥ 25 mg/dL. False-positive readings can be caused by metabolites of lodine (etodolac). |
Test is based on the coupling of bilirubin with diazotized dichloroaniline in an acidic medium. Test is less sensitive and specific than Ictotest Reagent Tablets. A positive test may be confirmed by Ictotest Reagent Tablets. To detect very small amounts of bilirubin in urine (eg, in the earliest phase of viral hepatitis), Ictotest Reagent Tablets should be used.
|Blood||Negative (<0.010 mg/dL hemoglobin or <3 RBC/mcL)c||0.015–0.062 mg/dL hemoglobin or 5–20 RBC/mcL||Test is equally sensitive to myoglobin and hemoglobin (including both intact RBC and free hemoglobin). Test is based on the peroxidase-like activity of hemoglobin. False-positive results can be caused by oxidizing contaminants (hypochlorite) and microbial peroxidase (urinary tract infection). Test sensitivity is reduced in urines with high specific gravity, captopril, or heavy proteinuria.|
|Nitrite||Negative||0.06–0.10 mg/dL nitrite ion||Test depends on the conversion of nitrate (derived from the diet) to nitrite by gram-negative bacteria in urine when their number is >105 /mL (≥ 0.075 mg/dL nitrite ion). Test is specific for nitrite. False-negative readings may occur with shortened urine time in the bladder (<4 hr) and can also be caused by ascorbic acid. Test sensitivity is reduced in urines with high specific gravity. A negative result does not rule out significant bacteriuria.|
|Leukocytes (esterase)||Negatived||5–15 WBCs/hpf||Indicator of urinary tract infection. Test detects esterases contained in granulocytic leukocytes. Test sensitivity is reduced in urines with elevated glucose concentrations ≥3 g/dL), or presence of cephalexin, cephalothin, tetracycline, or high concentrations of oxalate. False-positive results may occur with specimen contamination by vaginal discharge.|
NOTE: Fully automated urinalysis systems (either image- or flow cytometry-based) are now available in many clinical laboratories, so manual microscopy examination may not be performed routinely in a central laboratory.