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 Formed Elements

Urine microscopy is an important adjunct to the urinalysis. The findings of cells, casts or crystals can aid in urinalysis interpretation and in making a more accurate diagnosis. Since the formed elements are much more unstable than the chemical components of urine samples, careful attention should be paid to sample handling. Sample contamination must be minimized. High urine concentration and low urine pH help to preserve the formed elements. Therefore, the first morning void which is more likely to be acidic and concentrated should be used whenever possible. The sample should not be refrigerated as this leads to precipitation of phosphates and urates.[16]

Mohammad et al. reviewed the microscopic findings in the urine of 1661 pregnant women being routinely screened for asymptomatic bacteriuria.[67] Interestingly, although only 1.9% had culture-confirmed single organism bacteriuria, many patients had other findings. Leukocyturia was found in 13.5% of patients, 22.5% of patients showed the presence of bacteria, 2.5% of patients had RBCs, 3.5% of patients had crystals and 0.8% of patients had yeast species.[67] This demonstrates the nonspecific nature of urine microscopy, particularly in pregnancy when contamination may increase, and the importance of correct clinical interpretation of urinalysis reports.

In preparation for microscopy, a fresh urine sample should be centrifuged at approximately 2000 revolutions/min for 10 min. The resulting supernatant is carefully decanted and the remaining pellet re-suspended by gentle agitation. A single drop of this is then placed on a slide and covered. A bright-field microscope is most often used to examine urine; however, polarized light can be helpful to identify certain crystals.[16,68]

Red Cells

Visualizing RBCs on microscopy clarifies a positive dipstick test for hemoglobin. True hematuria is easily distinguished from myoglobinuria or hemoglobinuria in this way.[1,33]

Red cells seen on microscopy are generally categorized as isomorphic (normal shape) or dysmorphic (distorted shape). The proportion of either type is often used to identify the source of bleeding (glomerular or urologic).[20] The clinical utility of identifying dysmorphic RBCs has been examined in numerous studies. Most authors agree that the presence of dysmorphic RBCs is a reliable indicator of glomerular disease. In a meta-analysis of 21 published studies, Offringa and Benbassat calculated a weighted average sensitivity of 0.88 (95% CI: 0.86–0.90) and specificity of 0.95 (95% CI: 0.93–0.97) for the predictability of glomerular disease (e.g., IgA nephropathy or systemic lupus nephritis) based on the presence of dysmorphic RBCs.[69]

White Cells

The number of WBCs normally found in urine is controversial. It is generally accepted that, conservatively, more than one WBC per HPF is abnormal, although, some experts consider five or more to be indicative of pathologic leukocyturia.[16]

Differential diagnosis of leukocyturia is broad, as WBCs can enter from anywhere along the excretory system. When found in combination with other formed elements, such as casts, a glomerular source is more likely.[9] The presence of WBCs may be indicative of a UTI, but may also be present in many other conditions. Febrile illness, glomerulonephritis, and inflammatory processes both in and out of the pelvis (e.g., pelvic inflammatory disease) can all cause leukocyturia.[1] A positive urinalysis (greater than five WBC per HPF) may help differentiate pelvic inflammatory disease from appendicitis, as the latter is rarely associated with leukocyturia.[70] Isolated leukocyturia was also found to be predictive of the presence of a sexually transmitted disease (Trichomonas vaginalis, Neisseria gonorrheae or Chlamydia trachomatis) in a cross-sectional sample of 296 sexually active female patients aged 14–22 years presenting with urinary symptoms.[71]

The type of WBC can also be helpful. For example, neutrophils suggest infection but can also be indicative of specimen contamination with nearby vaginal secretions. Eosinophils are classically associated with hypersensitivity reactions, possibly drug-induced.[21]

Other cells

Many other cells can be seen in urine and are of variable clinical significance. Renal tubule epithelial cells can be oval to columnar in shape, depending on the segment of the tubule they come from. They are considered to be a marker of tubular damage. When seen with granular casts, renal tubule epithelial cells are highly predictive of acute tubular necrosis and can help to exclude prerenal acute kidney injury, seen in postoperative patients who may be hypovolemic secondary to blood loss and/or dehydration.[72]

Urothelial cells are derived from the epithelial lining of the urinary tract from the renal calyces to the bladder in women. They are further classified into two categories. Deep urothelial cells are smaller and are associated with urological disease, such as bladder carcinoma, ureteric stones or hydronephrosis. Superficial urothelial cells are more commonly seen and are larger. They are more suggestive of a UTI.[9]

Squamous cells are the largest of the cells seen in urinary sediment. They come from the urethra or external genitalia. When found in massive amounts, the specimen should be considered contaminated.[1,9,15]

Bacteria

Bacteriuria in asymptomatic women is considered significant when counts of a known urinary pathogen equal or exceed 105 colony-forming units/ml.[73] Although this standard has been extended to symptomatic patients, some literature suggests that low-count bacteriuria (102–105 colony-forming units) is important to recognize in patients presenting with dysuria and/or frequency. Stamm et al. found that the detection rates of coliforms in midstream urine specimens in symptomatic women increased from 51 to 88% when the criterion was lowered from ≥105 to ≥102 colony-forming units/ml.[74]

The prevalence of asymptomatic bacteriuria in the gynecologic population is approximately 10%.[75] This may increase to nearly 50% in postoperative patients requiring urinary catheterization for 24 h or more. In most cases, the presence of bacteria in an asymptomatic patient who is not pregnant and does not have an indwelling catheter does not require treatment, as it will spontaneously resolve in 73–85% of cases.[75]

By direct contrast, while the prevalence of asymptomatic bacteriuria is similar in the obstetric population, the natural history is much different. Pregnant women who are not treated are more likely to develop pyelonephritis (20–35%) and deliver a low birthweight infant (15%).[76] For this reason, screening for and treating asymptomatic bacteriuria has become a routine part of most antenatal care guidelines. A single urine culture before 20 weeks of gestation is recommended, and treatment should be based on the organism found. However, this standard was tested by McIsaac et al. in 1050 pregnant women; a single urine culture before 20 weeks of gestation missed more than 50% of the asymptomatic bacteriuria cases. A culture in each trimester identified most cases (87.8%).[77]

Casts

Casts are formed in the distal tubules and collecting ducts where Tamm–Horsfall proteins precipitate and entrap cells present in urine. Acidic urine and dehydration leading to concentrated urine favors the formation of casts. Identifying casts in urine sediment can provide important diagnostic information and may even be predictive of progression or regression of known renal disease.[78]

The two types of cast that can be seen in healthy individuals are hyaline and fine granular casts. They are usually associated with states of dehydration. Cellular casts are generally more helpful. For example, red cell casts are seen in glomerular disease and white cells casts are classically associated with glomerulonephritis. Renal tubular epithelial cell casts are always indicative of tubular damage. Both coarse granular casts and waxy casts are formed from the breakdown of cellular casts and therefore are indicative of renal pathology, possibly chronic or end-stage renal disease, but are nonspecific.[16]

Crystals

Many different types of crystals can be seen in urinary sediment. They can be classified as common, pathologic and drug-associated. The finding of calcium oxalate, calcium phosphate or uric acid crystals is common and often of little clinical significance. These crystals may reflect mild dehydration, ingestion of certain foods or increased interval between sample collection and examination. However, if these crystals are noted over multiple samples, one must consider the possibility of a metabolic disorder or acute renal failure due to toxicity.[9]

Many commonly prescribed drugs can produce crystalluria. Normally, drugs cause atypical or pleomorphic crystals that are very different in appearance to pathologic crystals. Ciprofloxacin, amoxicillin, acyclovir or indinavir may all cause crystalluria with birefringent, needle-like crystals.[9] Pathologic crystals include cholesterol crystals, which are found in association with marked proteinuria, and cysteine crystals, which are pathognomonic for the inherited condition cystinuria.[15]

Oversaturation of urine with crystals can lead to stone formation. Frequent stone formers in association with urinary symptoms and a history of recent or remote pelvic surgery with insertion of a foreign body such as a permanent surgical mesh or even an intrauterine device must alert the clinician to the possibility of foreign body erosion into the bladder[79,80] and evaluation via cystoscopy is warranted.

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