Urinalysis

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Urinalysis is the most common office laboratory test, with dipstick urinalysis most often used (Cherry et al., 2007). Urinalysis is inexpensive, noninvasive, easily carried out, and could indicate a number of urinary and systemic conditions. Performing urinalysis in otherwise healthy people as a preventive health service is commonplace. However, screening is not recommended in asymptomatic adults for many of the conditions that urinalysis can identify (e.g., asymptomatic bacteruria, bladder cancer; see later). Routine urinalysis lacks evidence of benefit, and guidelines recommend against routine urinalysis as part of adult or child preventive care, suggesting it only be done if clinically indicated (Grenz et al., 2009 a, 2009b; Stephens and Wilder, 2003). The American Academy of Pediatrics also no longer recommends urinalysis as part of routine pediatric prevention (AAP, 2007).

Although urinalysis can detect occult disease, most positive results will not yield this outcome; more patients would incur unnecessary or potentially dangerous medical evaluations than would benefit. Thus, the potential value is elusive while there is the possibility of harm. It is important, however, not to confuse broad population screening recommendations with those specific to certain patients or problems. For example, microalbuminuria screening is routine in diabetes care (ADA, 2009).

Physicians should be aware that there is a good chance of false-positive and false-negative results with dipstick tests. Thus, abnormal results are not necessarily indicative of disease and may need confirmatory testing (Gerber and Brendler, 2007; Simerville et al., 2005).

Specimen Preparation

A midstream clean-catch specimen is the recommended standard for urinalysis. Uncircumcised men should retract the foreskin before urinating. Urine should be examined immediately, if possible, and should be refrigerated if it will not be examined in 1 to 2 hours.

Inspection

Normal urine color varies from clear (dilute) to yellow to deep golden and cloudy. Many substances can cause urine color to appear abnormal (Table 40-1). Cloudy urine may be caused by infection (pyuria), but the most common cause is phosphaturia, in which phosphate crystals precipitate in alkaline urine. Microscopic analysis can differentiate these two entities. A strong or foul-smelling sample does not necessarily indicate infection. Urine odor may change because of dietary intake (e.g., asparagus), medications, illness, or concentration. Fecal odor suggests gastrointestinal-vesical fistula.

Dipstick Urinalysis

Specific Gravity

Urine specific gravity ranges from 1.001 to 1.035. It reflects the urine concentration and is a marker of hydration status. However, conditions affecting renal functions, such as chronic kidney disease and syndrome of inappropriate antidiuretic hormone secretion (SIADH), alter specific gravity and its relation to hydration. The glomerular filtrate has a specific gravity of 1.010, and urine with this fixed specific gravity may indicate renal dysfunction.

Average urinary pH is usually acidic, ranging from 5.5 to 6.5. It reflects the serum pH except in patients with renal tubular acidosis (RTA) (Simerville et al., 2005). These patients have

Table 40-1 Factors Affecting Urine Color

Color

Causative Factor

Colorless

Very dilute urine; overhydration

Cloudy, milky

Phosphaturia; pyuria; chyluria

Red

Hematuria; hemoglobinuria, myoglobinuria; anthocyanin in beets and blackberries; chronic lead and mercury poisoning; phenolphthalein (in bowel evacuants); phenothiazines (e.g., prochlorperazine [Compazine]); rifampin

Orange

Dehydration; phenazopyridine (Pyridium); sulfasalazine (Azulfidine)

Green-blue

Biliverdin; indicanuria (tryptophan indole metabolites); amitriptyline (Elavil); indigo carmine; methylene blue; phenols (e.g., IV cimetidine [Tagamet]; IV promethazine [Phenergan]); resorcinol; triamterene (Dyrenium)

Brown

Urobilinogen; porphyria; aloe, fava beans, rhubarb; chloroquine, primaquine; furazolidone (Furoxone); metronidazole (Flagyl); nitrofurantoin (Furadantin)

Brown-black

Alkaptonuria (homogentisic acid); hemorrhage; melanin; tyrosinosis (hydroxyphenylpyruvic acid); cascara, senna (laxatives); methocarbamol (Robaxin); methyldopa (Aldomet); sorbitol

From Hanno PM, Wein AJ. A Clinical Manual of Urology. Norwalk, Conn, Appleton-Century-Crofts, 1987, p 67.

alkaline urine because the kidneys cannot acidify urine. Type

1 RTA will always have alkaline urine, whereas urine in type

2 RTA may become acidic as the acidosis worsens. Urine infected with urease-producing organisms such as Proteus species becomes alkaline.

Blood

Dipstick urinalysis has a sensitivity of 91% to 100% and a specificity of 65% to 99% for microscopic hematuria (Grossfeld et al., 2001a). Myoglobin and hemoglobin can cause a false-positive dipstick reaction. Thus, heme-positive urine specimens require microscopic examination.

Glucose

Glucose in the urine (glycosuria) occurs when the glucose concentration of the glomerular filtrate exceeds the proximal tubule's ability to resorb it. The dipstick only reacts to glucose. A finding in uncontrolled diabetes mellitus, glycosuria occurs when the serum glucose level exceeds 180 mg/dL.

Ketones

Urinary ketone bodies are a main feature of diabetic ketoaci-dosis. They may also be found in starvation states and pregnancy. Dipsticks detect acetoacetic acid. False-positive results may occur with very concentrated or acidic urine.

Leukocyte Esterase

Leukocyte esterase is a substance produced by neutrophils that signifies possible pyuria. It is sensitive (72%-97%), but not very specific (41%-86%) for urinary tract infection (UTI). Urine contamination by vaginal cells is the most common reason for false-positive results. Patients with typical UTI symptoms, positive leukocyte esterase, and negative urine cultures should be evaluated for sexually transmitted infections (STIs) (Graham and Galloway, 2001).

Nitrite

The finding of nitrites in the urine is 92% to 100% specific for a UTI (Simerville et al., 2005). However, many patients with UTIs will not be nitrite positive (i.e., low sensitivity). Gram-negative coliform organisms convert urinary nitrates to nitrite, but not Staphylococcus saprophyticus or enterococci. The test loses accuracy with lower bacterial colony counts. The reagent is air sensitive, so a dipstick may yield false-positive results if strips in the container are not tightly sealed (Gallagher et al., 1990).

Protein

Urine dipsticks are very sensitive and specific for albuminuria. The reagent color change roughly corresponds to the protein concentration in the sample (Table 40-2). Very dilute, alkaline, or nonalbumin proteinuria may cause false-negative results. Microalbuminuria screening usually requires a separate dipstick designed specifically for this purpose. A urine protein electrophoresis is needed to evaluate nonalbumin proteinuria, such as globulinuria or Bence Jones protein.

Urobilinogen

Urobilinogen is a breakdown product of conjugated bilirubin. There is normally a small amount present, but elevations indicate possible hemolysis or liver disease. Conjugated bili-rubinuria is abnormal and signifies hepatic disease or biliary obstruction. Unconjugated bilirubin is not filtered by the glomerulus.

Urine Microscopy

To prepare a urine sample for microscopic analysis, centrifuge 10 mL of urine for 5 minutes at 2000 rpm. After centrifuga-tion, pour off the supernatant and resuspend the remaining sample (0.5-1.0 mL). Place a drop of this sample on the slide and focus up to high power. Samples showing probable skin or vaginal contamination should be retested or a catheter-ized specimen obtained (Grossfeld et al., 2001a). Sediment counts should be estimated as the average number of elements viewed per high-power field (hpf).

Red Blood Cells

Urine specific gravity may affect findings, because red cells may lyse at values lower than 1.007 (Vaughan and Wyker, 1971). Normal-appearing red blood cells (RBCs) suggest a lesion in the urinary tract, whereas dysmorphic RBCs are probably of glomerular origin (Fig. 40-1). Red cell casts are typically found at the edges of the glass coverslip and are highly suspicious for renal parenchymal disease.

White Blood Cells

Normal urine samples may show some white blood cells (WBCs) on high-power examination (Fig. 40-2). For women, fewer than 5 WBCs/hpf is normal, and for men fewer than 2 WBCs/hpf is normal (Simerville et al., 2005). Values in excess of these limits constitute microscopic pyuria. Purulent, cloudy urine with WBCs too numerous to count describes gross pyuria.

White cells are inflammatory markers, so infection cannot be confirmed by their presence or ruled out by their absence. Common causes of sterile pyuria include STIs, kidney stones, prostatitis, and urinary tract neoplasms. In children, pyuria may occur during a febrile illness, even if a UTI is not present (Graham and Galloway, 2001).

Casts

RBC casts signify a nephritic or vasculitic process. WBC casts are often considered pathognomonic for pyelonephritis, but may be found in other types of nephritis. Granular and waxy casts signify renal parenchymal disease. Hyaline casts may

Table 40-2 Dipstick Protein Findings

Dipstick Color

Estimated Corresponding Protein Level (mg/dL)

Yellow

Negative (no protein)

Yellow-green

Trace (10-20)

Green

1+ (30)

Dark green

2+ (100)

Green-blue

3+ (300)

Blue

4+ (>1000)

From Simerville JA, Maxted WC, Pahira JJ. Urinalysis: a comprehensive review. Am Fam Physician 2005;71:1153-1162.

be normal in concentrated specimens or a marker of renal infection or disease.

Crystals

Various crystal morphologies may be seen in urine samples (Fig. 40-3).

Urine Microscopic Wbcs

Figure 40-1 Urinalysis: dysmorphic red blood cells. (From Gerber GS, Brendler

CB. Evaluation of the urologie patient. in Wein AJ [ed]. Campbell-Walsh Urology, 9th ed, vol 1. Philadelphia, Saunders-Elsevier, 2007, p 106.)

Figure 40-1 Urinalysis: dysmorphic red blood cells. (From Gerber GS, Brendler

CB. Evaluation of the urologie patient. in Wein AJ [ed]. Campbell-Walsh Urology, 9th ed, vol 1. Philadelphia, Saunders-Elsevier, 2007, p 106.)

Urinary Epithelial (Urothelial) Cells

Transitional epithelial cells are normally found in microscopic specimens. They are uniform in shape and have a large, central nucleus. In contrast, squamous cells have irregular borders and a small nucleus. Squamous cells indicate skin contamination.

Bacteria

Bacteria may signify bacteruria, although in women vaginal contamination is also likely. A urine Gram stain is also helpful for identifying bacterial characteristics in urine samples in which infection is suspected.

Fungi

Yeast seen in urine specimens most likely indicates contamination from the skin or vagina. However, it may signify systemic Candida infection if found in a catheterized specimen (Graham and Galloway, 2001).

Trichomonads

These motile organisms signify a common STI and may be seen in urine samples. Treatment with metronidazole (2 g as a single dose or 500 mg twice daily for 7 days) is effective. Patients should be evaluated for other STIs.

Urinalysis Wbc
Figure 40-2 Urinalysis: white blood cells. (From Gerber GS, Brendler CB. Evaluation of the urologie patient. in Wein AJ [ed]. Campbell-Walsh Urology, 9th ed, vol 1. Philadelphia, Saunders-Elsevier, 2007, p 107.)

Crystals

B ^^^^

Cystine

Calcium Oxalate

C ^

D

Uric Acid Triple-Phosphate

(Struvite)

Figure 40-3 Crystal types found in urine samples. A, Cystine. B, Calcium oxalate. C, Uric acid stones are found in acidic urine. D, Triple-phosphate crystals are found in alkaline specimens. (From Gerber GS, Brendler CB. Evaluation of the urologic patient. In Wein AJ [ed]. Campbell-Walsh Urology, 9th ed, vol I. Philadelphia, Saunders-Elsevier, 2007, p I08.)

Uric Acid Triple-Phosphate

(Struvite)

Figure 40-3 Crystal types found in urine samples. A, Cystine. B, Calcium oxalate. C, Uric acid stones are found in acidic urine. D, Triple-phosphate crystals are found in alkaline specimens. (From Gerber GS, Brendler CB. Evaluation of the urologic patient. In Wein AJ [ed]. Campbell-Walsh Urology, 9th ed, vol I. Philadelphia, Saunders-Elsevier, 2007, p I08.)

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  • mathias scholz
    Why white blood cells are seen in urine microscopy?
    2 years ago

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