Urine Microscopy: Cells, Casts, Crystals, and Artifacts

From screening to microscopy

Many laboratories use a validated reflex algorithm to select specimens for microscopy. An order for microscopy, an abnormal screening result, an instrument flag, or a clinical setting named by the local protocol may trigger review. Such settings may include pregnancy, pediatric or older-adult care, diabetes, immunocompromise, and known renal disease. The criteria belong in the laboratory procedure and should reflect the population and the performance of the screening methods.^{11 Kouri TT, et al. The EFLM European urinalysis guideline 2023. 2024.}

Use the physical and chemical results to decide what to look for under the microscope. Then compare the findings for agreement, because several causes can produce the same screening result.

The Specimen Handling and Acceptability lesson covers routine collection, transport, preservation, and rejection rules. Sediment examination uses a well-mixed specimen within its validated stability period. Delay and dilute alkaline urine promote lysis of RBCs and WBCs and loss of hyaline casts. Refrigeration can precipitate urates or phosphates that obscure other particles. Bring a refrigerated specimen to the temperature required by the laboratory procedure and document any handling limitation.

Standardized sediment examination

A conventional concentrated-sediment method uses these linked steps:

1. Mix the specimen and transfer the procedure's exact starting volume. The European Federation of Clinical Chemistry and Laboratory Medicine (EFLM) guideline gives 5 to 12 mL as a conventional range. A traditional method may begin with 12 mL.
2. Centrifuge at the verified relative centrifugal force (RCF) and time. A common setting is approximately 400 × g for 5 minutes. The rotor radius determines the revolutions per minute: RCF = 1.118 × 10⁻⁵ × radius (cm) × RPM².
3. Remove supernatant to the defined final sediment volume in one continuous motion without disturbing the packed sediment. Resuspend the sediment completely. Dividing the starting volume by the final volume gives the concentration factor. In the 12 mL example, final volumes of 0.5 mL and 1.0 mL produce 24-fold and 12-fold concentration.
4. Load a fixed volume into the specified chamber or beneath the specified coverslip. A traditional slide method may use 20 µL under a 22 × 22 mm coverslip. Keep the chamber depth, coverslip area, and settling time constant.
5. Scan at low power, commonly a 10× objective and about 100× total magnification, for casts and overall distribution. Examine cells, crystals, and microorganisms at high power, commonly a 40× objective and about 400× total magnification.
6. Count the validated volume or number of fields and report in the unit defined by the procedure. A traditional field method averages RBCs and WBCs across at least 10 high-power fields (HPFs) and casts across at least 10 low-power fields (LPFs). Calibrated chambers and automated systems can report particles per microliter or per liter.

The starting volume, final volume, chamber, field count, and report categories form one method. Results from a different method are comparable only after verification. Centrifugation and supernatant removal can lose RBCs and WBCs and can fragment casts. Direct counting of uncentrifuged urine in a calibrated chamber can give a more accurate cell concentration, and concentration improves detection of rare casts and renal tubular epithelial cells.^{11 Kouri TT, et al. The EFLM European urinalysis guideline 2023. 2024.}

Automated particle analyzers use flow methods, digital imaging, or both. The laboratory validates the instrument's reportable categories and sets manual-review rules for flags, unusual images, rare particles, dysmorphic RBCs, nonsquamous epithelial cells, casts, crystals, and discrepancies with the physical or chemical findings.^{11 Kouri TT, et al. The EFLM European urinalysis guideline 2023. 2024.}

Optical methods

Most unstained sediment particles have little contrast. Reduced illumination improves bright-field visibility. Phase-contrast microscopy improves detection and discrimination of low-refractive-index particles and is especially useful for hyaline casts, cell detail, and dysmorphic RBCs.

A concentrated Hansel- or Wright-stained preparation can demonstrate urine eosinophils. Sensitivity and specificity are poor, no threshold confirms acute interstitial nephritis, and current EFLM guidance finds routine reporting clinically unhelpful. Targeted workflows may use Gram stain, Prussian blue, dark-field microscopy, fluorescence, or interference-contrast optics for a specific organism, pigment, antigen, antibody, or structural question. These methods follow separate validated laboratory procedures.^{11 Kouri TT, et al. The EFLM European urinalysis guideline 2023. 2024.}

Cells

Identify a cell from its size, contour, nucleus, internal texture, and focal plane, together with specimen concentration and chemical findings. A single feature is rarely sufficient. The figure compares source photomicrographs. Preparation, stain, and optical setting vary by source and change appearance.

Microorganisms, sperm, and mucus

Bacteria in a voided specimen may represent contamination, colonization, or infection. Report the microscopic finding according to the validated category and correlate it with WBCs, chemistry, collection quality, symptoms, and culture. Brownian motion can resemble bacterial motility.

Yeast appear as refractile oval forms with budding or pseudohyphae. RBCs, oil droplets, and starch can mimic yeast. Candiduria often reflects colonization, and urine findings alone cannot reliably separate colonization from infection.^{44 Pappas PG, et al. Clinical practice guideline for the management of candidiasis. 2016.}

Motile Trichomonas vaginalis trophozoites are pear-shaped flagellates with an undulating membrane. Motility falls as the specimen stands. A urine wet preparation has limited sensitivity. Report a finding under the laboratory's sexually transmitted infection confirmation and reporting procedure.^{55 Centers for Disease Control and Prevention. DPDx: Trichomoniasis.}

Schistosoma haematobium eggs are large, approximately 110 to 170 by 40 to 70 µm, and have a terminal spine. Travel or residence in an endemic area is essential history, and the finding requires parasite confirmation. Enterobius vermicularis eggs in urine usually reflect fecal or perineal contamination. The perianal tape test is the preferred diagnostic specimen.^{6,76 Centers for Disease Control and Prevention. DPDx: Schistosomiasis. 2024.7 Centers for Disease Control and Prevention. DPDx: Enterobiasis.}

Spermatozoa have an oval tapered head and a long tail and are usually nonmotile in urine. They may be relevant to infertility or retrograde-ejaculation testing. Reporting depends on the specimen, patient population, and laboratory policy. Mucus forms long, pale, irregular threads and is usually insignificant. Its tapered or frayed ends and variable width help separate it from a cast.

Casts

The printable cast guides show brightfield photographs and phase-contrast photographs alongside identifying features and look-alikes.

Casts form in distal tubules and collecting ducts when fibrils of uromodulin (the protein secreted by tubular cells that forms the cast matrix) gel into a cylindrical matrix. Urinary stasis, acidic pH, and favorable ionic conditions promote formation. Cells, filtered proteins, lipids, microorganisms, pigments, and crystals can become embedded in that matrix. Scan at low power to find casts, and use high power to identify their contents. A true cast has a matrix, generally with parallel sides and rounded or broken ends.^{1,21 Kouri TT, et al. The EFLM European urinalysis guideline 2023. 2024.2 Cavanaugh C, et al. Urine sediment examination in the diagnosis and management of… 2019.}

Crystals

Crystals form when a substance exceeds its solubility. Urine pH, concentration, temperature, storage, diet, medication exposure, and the amount of the substance all affect precipitation. Transient crystals are common. Abundant, recurrent, atypical, drug-associated, stone-associated, or acute kidney injury-associated crystalluria requires closer identification. That identification combines shape, color, pH, polarization, solubility, medication history, and confirmatory chemistry or stone analysis.^{1,21 Kouri TT, et al. The EFLM European urinalysis guideline 2023. 2024.2 Cavanaugh C, et al. Urine sediment examination in the diagnosis and management of… 2019.}

Contaminants and artifacts

Artifacts are often highly refractile or appear in a different focal plane from the sediment. Their shape, optical behavior, and the surrounding chemical and microscopic findings guide exclusion.^{1,21 Kouri TT, et al. The EFLM European urinalysis guideline 2023. 2024.2 Cavanaugh C, et al. Urine sediment examination in the diagnosis and management of… 2019.}

Reporting and interpretation

Report the method's unit and category, the particle identity, and any permitted qualifier. Quantitative cell and cast results support comparison over time only when the specimen type and method remain comparable. Crystals, microorganisms, mucus, and artifacts often use validated ordinal categories because visual microscopy has limited counting precision for these particles. Common local terms include rare, few, moderate, and many, or 1+ through 4+. The procedure defines each category.

The American Society for Clinical Pathology (ASCP) Board of Certification (BOC) exam lists reference ranges of RBCs 0 to 3/HPF, WBCs 0 to 8/HPF, hyaline casts 0 to 2/LPF, and epithelial cells 0 to 5/HPF. These values are examination references. The laboratory establishes clinical reference intervals for its specimen and method. The ASCP BOC Examination Reference Ranges lesson contains the complete official table.^{88 American Society for Clinical Pathology… Medical Laboratory Scientist, MLS(ASCP) and MLS(ASCPi)… 2026.}

Sediment patterns help localize a process. Dysmorphic RBCs and RBC casts support glomerular bleeding. WBC casts support renal inflammation or infection. Renal tubular epithelial cells and their casts support tubular injury. Fatty casts and oval fat bodies support lipiduria with heavy protein loss. Granular, waxy, and broad casts support increasing stasis or parenchymal injury. Combine each interpretation with specimen quality, chemistry, culture, kidney function, and the clinical setting before you report the pattern.