A urinalysis sometimes reveals the presence of cells that line the urinary tract, known as transitional cells or urothelial cells. These specialized epithelial cells are routinely examined as part of a urine sediment analysis. Finding a few of these cells is considered normal due to the natural turnover of the body’s tissues. However, an elevated count or abnormal characteristics can indicate an underlying medical condition.
The Origin and Function of Urothelial Cells
Transitional cells line the inner surfaces of the entire urinary tract, creating a protective barrier known as the urothelium. This lining extends from the renal pelvis, through the ureters, into the bladder, and down to the proximal urethra. The term “transitional” refers to the unique ability of these cells to change shape and arrangement.
When the bladder is empty, the urothelial cells appear large and cuboidal, but they flatten and stretch as the bladder fills to accommodate the increasing volume of urine. The outermost layer consists of large, dome-shaped umbrella cells, which form an impermeable seal. This barrier prevents toxic components of urine, such as acid and urea, from diffusing back into the body’s tissues.
The urothelium consists of multiple layers, including basal cells, intermediate cells, and superficial umbrella cells. Basal cells continuously generate new cells, leading to the natural shedding of older cells from the surface. This normal cell turnover accounts for the routine presence of a small number of transitional cells in a sediment analysis.
Distinguishing Normal Shedding from Clinical Significance
The presence of a few transitional cells in a microscopic urine examination is considered a benign finding. Laboratory professionals quantify the cell count using descriptive terms like “few,” “moderate,” or “many.” A report indicating “few” transitional cells is usually interpreted as normal and requires no further investigation.
The quantity of cells becomes clinically significant when the report indicates “moderate” or “many” cells. An increased shedding rate suggests the urothelium is being irritated, inflamed, or damaged, causing cells to detach more rapidly than normal. Healthcare providers correlate this result with the patient’s symptoms and other lab findings. Significance also increases if the cells are found in clusters, suggesting a deeper layer of the urothelium has been affected.
Benign Causes of Increased Transitional Cell Presence
A high number of transitional cells frequently points to common conditions causing irritation or inflammation of the urinary tract lining. Urinary tract infections (UTIs) are a primary cause, where bacterial inflammation triggers the rapid sloughing of urothelial cells. In these cases, the increased cell count is often accompanied by white blood cells and bacteria in the urine sediment.
Physical trauma, such as the passage of kidney or bladder stones, is another frequent benign cause, as the stones scrape the urothelium. Recent medical procedures, like catheterization or cystoscopy, can also cause mechanical trauma resulting in a temporary elevation of transitional cells. These cells are considered reactive, meaning their appearance reflects an inflammatory response. Chronic inflammatory conditions, such as interstitial cystitis, can also lead to persistently high counts.
Interpreting Atypical Cells and Follow-Up Investigation
The most concerning finding is when transitional cells are described as “atypical” or “suspicious” on a urine cytology report. Atypical urothelial cells show morphological changes, such as an enlarged nucleus or irregular shape, suggesting an abnormality but not meeting the definitive criteria for cancer. This intermediate diagnosis is challenging because inflammation from a UTI or kidney stone can cause reactive changes that mimic true cellular atypia.
The diagnosis of atypical cells requires immediate follow-up evaluation because a significant percentage of patients are eventually diagnosed with urothelial carcinoma, the most common type of bladder cancer. The risk is elevated in patients who also present with hematuria, a history of smoking, or prior bladder cancer. Pathologists may subclassify the finding, with “atypical urothelial cells cannot exclude high-grade” carrying a higher concern for malignancy.
The standard investigation following an atypical result includes a comprehensive physical examination and often a cystoscopy for direct visual inspection of the bladder lining. Imaging studies, such as a CT scan or MRI, may also be used to evaluate the upper urinary tract. Ancillary tests like molecular analyses or fluorescence in situ hybridization (FISH) are sometimes employed to look for specific genetic changes in the cells.