Contaminants

Let’s get into the nitty-gritty of what doesn’t belong. This is a crucial skill because your ability to recognize a contaminant directly impacts the quality and validity of the entire urinalysis. A good laboratory scientist acts as a gatekeeper, ensuring that the results reported truly reflect the patient’s condition, not just what fell into the collection cup

First, let’s make a critical distinction:

• Contaminant: A real, physical substance that is present in the specimen cup but did not originate from the patient’s urinary tract. Think of it as evidence from outside the “crime scene” that was accidentally introduced
• Artifact: An element that appears during the process of preparing or viewing the slide but wasn’t in the original sample. This includes things like air bubbles, oil droplets from the microscope, or scratches on the glass slide. These are illusions of the process itself

Fibers and Hairs

These are probably the most common contaminants you’ll encounter. They can come from clothing, diapers, toilet paper, or can be airborne in the lab

• Description
  • Fibers: Usually long, flat, and highly refractile with dark edges. They are often variable in width and may have distinct colors (like a blue cotton fiber). They can look like a cast at first glance, but they are a classic cast “look-alike.”
  • Hairs: Have a distinct structure with a visible shaft and, occasionally, a root or follicle
• How to Differentiate from a Cast: This is a key skill. Remember that casts have parallel sides, a uniform width, and rounded ends. Fibers are irregular, not uniform in width, and often have sharp or frayed ends
• Clinical Significance: Absolutely none, but they are a major nuisance. The goal is to recognize them for what they are and not misidentify them as a pathological element like a cast. They are not reported

Starch and Powders

• Description: Starch granules are typically round or oval and can be slightly irregular in shape. The most characteristic feature is a central indentation or “y-slit.” The most important feature to remember is that starch granules are birefringent under polarized light, producing a distinct “Maltese cross” pattern
• Origin: This can come from powdered surgical gloves (less common now with the switch to powder-free gloves, but they still exist), body powders, or feminine hygiene products
• Clinical Significance: No direct clinical significance. However, their ability to polarize light and form a Maltese cross means they are a critical look-alike for the pathologically significant oval fat bodies or free fat globules seen in nephrotic syndrome
  • Differentiation: Starch granules are generally more uniform and regular in their Maltese cross pattern compared to the variable size and appearance of fat droplets. Context is key—you wouldn’t expect to see starch alongside heavy proteinuria and fatty casts

Fecal Contamination

This is a very significant finding that has major implications for the validity of the sample

• Description: Can appear as a messy collection of amorphous, brownish material. The key identifiers are elements that are clearly from the digestive tract:
  • Plant fibers: Often have a spiral structure
  • Meat fibers: Have visible striations
  • Brown pigment and a massive load of mixed bacteria.
  • Occasionally, you may even see a parasite egg like Enterobius vermicularis (pinworm), which is a definitive sign of fecal contamination
• Origin: Improper sample collection. Rarely, it can be due to a vesicointestinal fistula (a direct connection between the bladder and the intestine), which is a serious pathologic condition, but the material itself is still considered a contaminant from the GI tract
• Clinical Significance: This is a critical finding. A urine specimen contaminated with feces is unsuitable for analysis, especially for urine culture. It introduces a massive, non-urinary bacterial load that makes interpreting a culture impossible. The correct action is to note the contamination and recommend that the provider obtain a new, properly collected specimen

Vaginal Contamination

This is perhaps the most common reason for a “dirty” looking urine sediment and requires careful interpretation

• Description: This isn’t one single element but a constellation of findings:
  • Numerous Squamous Epithelial Cells: This is the #1 indicator of vaginal contamination. While a few can be normal, seeing “moderate” or “many” suggests the sample is not a clean-catch
  • Mucus: Often seen in large amounts
  • “Mixed flora” of bacteria: A variety of bacterial shapes and sizes without a predominant type
  • Clue Cells: This is a squamous epithelial cell completely covered in bacteria (typically Gardnerella vaginalis). The cell border becomes indistinct, shaggy, and granular. Finding a clue cell is highly suggestive of bacterial vaginosis, a vaginal condition, not a UTI
• Origin: Normal vaginal flora and secretions introduced into the cup during urination
• Clinical Significance: Extremely important for interpretation. It tells you that any bacteria or WBCs present may not be from the urinary tract. It calls into question the validity of a positive Leukocyte Esterase or a positive urine culture. When you see evidence of significant vaginal contamination, it’s good practice to add a comment to your report, such as “Numerous squamous epithelial cells present, suggesting possible vaginal contamination; interpret bacterial and WBC results with caution.” This provides crucial context for the physician

Putting It All Together: Separating the Signal from the Noise

When you look at a urine sediment, you are not just a scientist; you are a quality control expert and a gatekeeper. Your primary mission is to distinguish the true pathological “signal” from the environmental “noise.” This noise comes in the form of contaminants—the real-world clutter that has found its way into the specimen cup

Your expertise is proven by winning these classic battles of identification: * Recognizing that a long, refractile fiber: is an imposter and not a clinically significant cast * Knowing that the starch granule: with its deceptive “Maltese cross” is a look-alike for a pathologic oval fat body * Understanding that a flood of bacteria: accompanied by numerous squamous epithelial cells tells a story of vaginal contamination, not necessarily a UTI

Identifying these contaminants is more than just clean-up work. It is a critical skill that prevents misdiagnosis, protects the integrity of the result, and ensures that the story you report is the patient’s story—and nothing else