A small number of transitional cells showing up in a routine urine test is usually normal and not a cause for alarm. These cells line your urinary tract, and a few naturally shed into urine every day as part of ordinary tissue turnover. What gets a doctor’s attention is when the count is unusually high, when the cells look abnormal under a microscope, or when they appear alongside other worrying signs like blood in the urine. In those cases, transitional cells can be an early clue to conditions ranging from a simple urinary tract infection to bladder cancer.
What Transitional Cells Actually Are
Your urinary tract, from the inside of the kidneys down through the ureters, bladder, and the upper part of the urethra, is lined with a specialized tissue called urothelium, sometimes called transitional epithelium. The name “transitional” comes from the fact that these cells can change shape: they stretch flat when the bladder is full and bunch up into a rounder shape when it is empty. This flexibility allows the bladder to expand and contract without tearing its lining.
Because this lining is in constant contact with urine, cells are regularly shed from its surface and replaced by new ones growing from below. When a lab technician spins down a urine sample and examines the sediment under a microscope, finding a handful of transitional epithelial cells is expected. Most urine reports distinguish three types of epithelial cells: squamous cells (from the lower urethra or external genitalia, very common and almost always insignificant), renal tubular cells (from the kidneys, potentially more concerning), and transitional cells (from the bladder and ureters). It is the transitional cells that matter most when doctors are thinking about diseases of the bladder or upper urinary tract.
When a Few Transitional Cells Are Perfectly Normal
A low number of transitional cells in urine, sometimes reported as “few” or “rare” on a lab slip, reflects the normal renewal process of the bladder lining. Your body is constantly replacing old cells with new ones, and the old ones have to go somewhere. They end up in your urine.
Several everyday factors can bump the number up slightly without anything being wrong. Vigorous exercise, mild dehydration that concentrates urine, or even how the sample was collected can affect the count. In pregnant women near term, for example, pressure on the bladder and changes in urinary flow can alter what shows up in sediment. The presence of transitional cells in pregnancy samples has been studied as a potential marker for underlying urinary issues, though a modest number on its own is not diagnostic of any particular problem.
The key point is context. If a urine sample shows a few transitional cells with no blood, no bacteria, and no unusual cell shapes, your doctor will almost certainly move on without ordering further tests.
When Higher Numbers Point to a Problem
A higher-than-expected count of transitional cells tells your doctor that something is irritating or damaging the bladder lining faster than normal. The causes range from mild to serious.
- Urinary tract infections: Bacterial infections inflame the bladder lining, causing more cells to shed. UTIs are the most common benign reason for elevated transitional cells, and the count typically drops back to normal once the infection clears.
- Kidney stones: Stones scraping along the ureter or sitting in the bladder mechanically dislodge cells from the lining.
- Catheter use: An indwelling catheter constantly rubs against the urothelium, leading to increased shedding. This is routine and expected in catheterized patients.
- Recent procedures: A cystoscopy, where a camera is threaded into the bladder, can dislodge transitional cells and temporarily make urine cytology look more cellular than usual. One study of over 150 patients found that urine cytology results remained consistent before and after cystoscopy when no tumor was present, suggesting the procedure itself does not create false-positive cancer readings in most cases, though the cell count itself may tick up briefly.
Inflammation from medications, radiation therapy to the pelvis, or chronic conditions like interstitial cystitis can also increase the shedding rate. In all of these benign situations, the transitional cells themselves look normal under the microscope. They are just more numerous than expected.
Atypical Cells and the Question of Cancer
The real concern arises when transitional cells do not just increase in number but also look abnormal. When a pathologist examines urine cytology and finds cells with irregular nuclear membranes, darkly staining nuclei, coarse chromatin, or unusual clustering patterns, the report may use terms like “atypical urothelial cells” or flag the sample as suspicious for malignancy. This is where the conversation shifts from routine finding to active investigation.
Transitional cell carcinoma, now more commonly called urothelial carcinoma, is the most common type of bladder cancer. It arises from the same transitional cells that line the urinary tract, so cancerous versions of these cells can shed into urine before a patient has any noticeable symptoms.1PubMed. Sensitive detection of transitional cell carcinoma of the bladder by microsatellite analysis of cells exfoliated in urine That makes urine cytology a potentially powerful early warning system, though it is far from perfect.
Standard urine cytology is quite good at catching high-grade cancers, where the cells look dramatically abnormal. It is less reliable for low-grade tumors, where cancerous cells can look deceptively similar to normal or mildly irritated ones. Researchers have found that features like papillary cell clusters, nuclear membrane irregularity, and very dark “India-ink type” nuclei show up more frequently in truly malignant samples, while features like cytoplasmic vacuolization tend to appear in reactive but non-cancerous samples.2PubMed Central. Diagnostic significance of atypical category in the voided urine samples: A retrospective study in a tertiary care center But distinguishing early-stage cancer from inflammation-related changes remains one of the trickiest calls in urine pathology.
How Labs Distinguish Harmless Cells From Worrisome Ones
The distinction between reactive transitional cells (irritated but not cancerous) and low-grade urothelial carcinoma cells is not always obvious, even to experienced pathologists. Both can show enlarged nuclei and slight irregularities. This gray zone is a major source of ambiguous urine cytology reports, and it is the reason why a single borderline result rarely leads to a definitive diagnosis on its own.
Researchers have worked on making this distinction more objective. Computer-assisted measurements of cell features, known as morphometry, can quantify differences that the human eye struggles to resolve. Studies have identified specific ratios involving nuclear diameter and area relative to cell size that reliably differ between benign reactive cells and low-grade carcinoma cells.3Morphologia. The importance of morphometric parameters in differentiating benign/reactive urothelial cells from low-grade urothelial carcinoma: computer-assisted study on urine specimens High-grade urothelial carcinoma cells tend to have even more dramatically enlarged and irregular nuclei compared to atypical cells that fall into an uncertain category.4PubMed Central. Utility of Image Morphometry in the Atypical Urothelial Cells and High-Grade Urothelial Carcinoma Categories of the Paris System for Reporting Urinary Cytology
These measurement-based approaches are promising but not yet standard practice in every lab. Most clinical decisions still rest on the pathologist’s trained eye, guided by standardized reporting systems like the Paris System for Reporting Urinary Cytology, which sorts findings into categories ranging from clearly negative to clearly malignant, with several shades of uncertainty in between.
What Happens After Atypical Cells Are Found
If your urine cytology comes back with atypical transitional cells, the next steps depend on how suspicious the cells look and what other symptoms you have. A mildly atypical result in someone with an obvious UTI may simply prompt a repeat test after the infection is treated. A strongly atypical result, or one in a patient with risk factors like smoking or a history of bladder cancer, triggers a more urgent workup.
The standard initial investigation combines cystoscopy and imaging. Cystoscopy lets the urologist directly visualize the inside of the bladder, and urine cytology paired with cystoscopy has long been the cornerstone of bladder cancer detection and surveillance.5PubMed Central. Urine cytology and adjunct markers for detection and surveillance of bladder cancer If cystoscopy reveals a visible tumor, a biopsy settles the question. If the bladder looks clean but the cytology is persistently abnormal, the concern shifts to the upper urinary tract, meaning the ureters or kidneys.
A striking case report illustrated this scenario: a patient’s routine urine sediment revealed atypical cells with irregular nuclear membranes and coarse chromatin. Cystoscopy showed nothing wrong in the bladder, but CT imaging uncovered a large mass in the kidney’s renal pelvis, ultimately confirmed as high-grade upper tract urothelial carcinoma.6PubMed. When urinalysis speaks first: Unexpected atypical urothelial cells revealing upper tract urothelial carcinoma This underscores why atypical cells should not be dismissed simply because the bladder looks normal on cystoscopy.
Bladder Cancer Versus Upper Tract Cancer
The same type of transitional cell cancer can arise anywhere along the urinary tract’s lining, but where it appears matters for prognosis and treatment. Bladder urothelial carcinoma is far more common than upper tract urothelial carcinoma (tumors in the renal pelvis or ureters), which accounts for only a small fraction of all urothelial cancers.
Despite sharing the same cell type of origin, these two locations behave differently enough that researchers increasingly treat them as related but distinct diseases. They share some risk factors, like smoking and certain industrial chemical exposures, but upper tract tumors have additional specific associations, including certain hereditary cancer syndromes and exposure to particular plant-derived toxins.7PubMed Central. Why are upper tract urothelial carcinoma two different diseases? Upper tract cancers are also harder to diagnose before surgery because the ureters and renal pelvis are difficult to biopsy accurately, which sometimes leads to more aggressive surgical decisions than the tumor ultimately warranted.
From the patient’s perspective, the practical takeaway is this: if atypical transitional cells keep showing up in your urine and your bladder checks out fine, the investigation should not stop there. The cells had to come from somewhere, and the upper tract needs to be evaluated with imaging or ureteroscopy.
Smoking and Your Risk of Transitional Cell Cancer
If there is one modifiable risk factor that dominates the conversation around bladder cancer, it is smoking. Cigarette smoke contains dozens of carcinogens that are filtered through the kidneys and concentrated in urine, where they sit in prolonged contact with the transitional cell lining of the bladder.
A large study examining smoking and bladder cancer subtypes found that risk increased with both the number of cigarettes smoked per day and the number of years spent smoking. People who smoked heavily, at 40 or more cigarettes daily, had roughly seven times the risk of developing muscle-invasive bladder tumors compared to nonsmokers, while their risk for low-grade superficial tumors was about three times higher.8PubMed Central. Cigarette Smoking and Subtypes of Bladder Cancer The steeper risk for more aggressive cancers is worth noting: smoking does not just increase the odds of getting bladder cancer, it shifts the odds toward the more dangerous forms of it.
Quitting helps. The same study showed that stopping smoking reduced the risk of all subtypes of bladder cancer, with the benefit growing the longer you stay smoke-free. This is useful information for anyone who sees atypical transitional cells on a urine report and smokes, because it means the single most effective thing you can do to reduce your future risk is also the most obvious one.
Molecular Tests That Go Beyond Looking at Cells
Traditional urine cytology depends on a pathologist’s ability to spot abnormal-looking cells, which works well for high-grade cancers but misses a meaningful proportion of low-grade and early-stage tumors. This limitation has driven the development of molecular tests that can detect cancer-associated changes even when the cells look relatively normal under the microscope.
One approach uses microsatellite analysis, which examines DNA from cells shed in urine for patterns of genetic instability associated with cancer. In one study, this method identified about 97% of patients with bladder tumors, compared to 79% detected by conventional cytology. The improvement was especially pronounced for low-grade and low-stage tumors, exactly the cases where standard cytology struggles most.1PubMed. Sensitive detection of transitional cell carcinoma of the bladder by microsatellite analysis of cells exfoliated in urine
Another technique, fluorescence in situ hybridization (FISH), detects chromosomal abnormalities in urine cells. When FISH was combined with standard cytology, the pairing caught tumors that cytology alone had missed and reduced the rate of undetected high-grade cancers by over 60%.9PubMed. Combined application of cytology and molecular urine markers to improve the detection of urothelial carcinoma Other molecular markers, including one that measures a gene called survivin expressed at high levels in bladder cancer cells, have also shown the ability to distinguish cancer patients from those with benign bladder inflammation.10PubMed. Expression of survivin mRNA in urine exfoliated cells of patients with bladder transitional cell carcinoma detected by real-time PCR
None of these molecular tests have fully replaced traditional cytology, but they are increasingly used alongside it, particularly for patients under surveillance after a previous bladder cancer diagnosis. If your doctor orders a urine test that goes beyond basic cytology, these are the kinds of tests they may be adding.
Why How Your Sample Is Handled Matters
One aspect of urine testing that patients rarely think about, but that can directly affect results, is what happens to the sample between collection and analysis. Urine is not a friendly environment for cell preservation. It is warm, often acidic, and full of enzymes that break cells down. Once cells start degenerating, they can look artificially abnormal, potentially mimicking the features of cancer, or they can break apart entirely and become unreadable, causing the lab to miss real abnormalities.
Research has confirmed that any delay in fixing a urine sample for cytology significantly affects cell preservation, and that these degenerative changes can contribute to both false-positive and false-negative results.11PubMed Central. The consequence of delayed fixation on subsequent preservation of urine cells The recommendation from researchers is that urine samples intended for cytology should be fixed with a preservative immediately after collection. When proper preservation techniques are used, such as specific fixatives stored at cold temperatures, transitional epithelial cells can remain stable in count and appearance for up to nine days.12CrossRef API / Tropical Health and Medical Research. Optimization of Urine Storage with the Addition of 40% Formalin at a Temperature of 2–8°C: Stability of the Amount and Morphology of Sediment Elements
What this means for you: if your doctor sends you for a urine cytology test, the sample should go to the lab quickly. If your clinic uses a preservative vial for cytology specimens, use it as directed. A sample sitting at room temperature for hours before processing is more likely to produce an unreliable or ambiguous result, which could mean either unnecessary worry or a missed finding. If you receive an equivocal cytology report and the sample was collected under less-than-ideal conditions, a repeat test with proper handling is reasonable before moving to more invasive investigation.
Transitional Cell Tumors in Animals
Urothelial carcinoma is not exclusive to humans. Dogs, particularly certain breeds, develop transitional cell carcinoma of the bladder, typically in the trigone region near where the ureters connect. A documented case in a 14-year-old dog that had been treated repeatedly for presumed urinary infections before a bladder mass was eventually found on ultrasound illustrates how these tumors can mimic the symptoms of common benign conditions.13PubMed Central. Transitional cell carcinoma of the urinary bladder in a 14-year-old dog In veterinary medicine, as in human medicine, the diagnosis often comes later than it should because the early symptoms of blood in the urine and recurrent infections are easily attributed to less serious causes. Research into canine transitional cell carcinoma has contributed to understanding of the disease in both species, as the tumors share biological similarities that make dogs useful models for testing new treatments.