Are Atypical Urothelial Cells Cancer?

Atypical urothelial cells on a urine cytology report are not, by themselves, a diagnosis of cancer. The label means a pathologist saw cells that look abnormal enough to flag but not abnormal enough to call malignant outright. Think of it as a yellow light: something is off, but the test cannot tell you whether that something is a tumor, an infection, a kidney stone irritating tissue, or a reaction to a prior treatment. Depending on the study and the population, somewhere between roughly one in six and one in three people with this finding eventually turn out to have a malignancy, so the result always warrants further investigation.

What the Report Actually Says

When a lab examines your urine under a microscope, the pathologist is looking at cells that have shed from the lining of the urinary tract. Most of the time, those cells look clearly normal or clearly cancerous. A small percentage fall into an in-between zone where the cells show some worrisome features but do not check every box for a cancer diagnosis. That gray zone gets the label “atypical urothelial cells,” often abbreviated AUC.

The reason the category exists is practical: inflammation, urinary stones, certain medications, and prior procedures can all make normal cells look strange under a microscope, mimicking some features of cancer without actually being cancer.¹ Rather than force every ambiguous case into a “positive” or “negative” bin and risk either alarming someone unnecessarily or missing a real tumor, labs created this middle category. Some institutions go a step further, splitting AUC into subcategories like “atypical, favor reactive” and “atypical, unclear if reactive or neoplastic,” though that practice is not universal.²

How Often AUC Turns Out to Be Cancer

The numbers vary by study, patient population, and whether the researchers counted only people who went on to have a biopsy or everyone who received the AUC diagnosis. That distinction matters a lot, because patients who proceed to biopsy are usually the ones with additional risk factors, which skews the cancer rate upward in that subgroup.

A Finnish study that tracked all AUC cases over two years found an overall malignancy rate of about 30%. Among those who actually had tissue confirmed by biopsy, the rate climbed to roughly 63%.³ A separate study of patients who had no prior history of bladder cancer found a similar pattern: about a third of AUC cases had biopsy follow-up, and among those biopsied, roughly 65% turned out positive for malignancy.⁴ On the lower end, a study evaluating a urine biomarker test found that about 17% of patients with atypical cytology were ultimately diagnosed with urothelial carcinoma.⁵

So if you have just received an AUC result, the honest range is roughly 17% to 30% for the chance that cancer is eventually found, depending on your risk profile and what other tests show. That also means a majority of AUC patients do not have cancer, which is worth keeping in perspective while you await further workup.

Why Atypical Cells Appear Without Cancer

Several benign conditions can make urothelial cells look suspicious enough to land in the atypical bucket. Understanding these helps explain why the false-alarm rate is substantial.

Urinary tract infections and inflammation: An active infection or chronic irritation causes cells to swell and develop darker nuclei, features a pathologist might interpret as mildly atypical.
Kidney or bladder stones: Stones physically abrade the urinary lining, producing reactive cellular changes that overlap with early tumor features.
BCG therapy: Patients treated with intravesical BCG for prior bladder cancer frequently shed cells that show reactive atypia and degeneration, both of which are strongly associated with false-positive cytology results.⁶
Instrumentation: Catheterization, cystoscopy, or ureteral stents can traumatize cells and make them look abnormal on cytology, even when no disease is present.

The adoption of a standardized reporting system, called the Paris System, actually increased the rate of atypical diagnoses in benign cases by about eightfold at one institution, from 3% to over 24%.⁷ That sounds alarming, but it reflects tighter definitions catching more borderline cases rather than more disease existing. The trade-off is that more patients receive an unsettling result they then need to follow up on, even when nothing harmful is present.

How Voided Versus Instrumented Samples Compare

The type of specimen can influence how likely it is to be called atypical. A voided sample (you urinate into a cup) and an instrumented sample (collected via catheter or during a procedure) go through different handling, and instrumented specimens may contain cells that were mechanically dislodged or traumatized. One large review of over 5,000 specimens found that atypical diagnoses were rendered in about 7% of voided samples and about 8% of instrumented ones, a modest but real difference.⁸ If your atypical result came from a catheterized or procedure-based sample, your clinician will factor that context in when deciding how aggressively to pursue follow-up.

What Happens Next After an AUC Finding

An atypical result on urine cytology rarely stands alone as the only test. The standard next step is cystoscopy, a procedure in which a thin camera is threaded through the urethra to visually inspect the bladder lining. Many patients also undergo imaging of the upper urinary tract, typically a CT scan with contrast, to check the kidneys and ureters for abnormalities.⁹ If something suspicious is seen on cystoscopy, a biopsy is taken during the same procedure to get a definitive tissue diagnosis.

In a small series of patients whose urine showed atypical cells, cystoscopy and imaging identified bladder tumors in seven out of ten cases that went to full workup.¹⁰ That series was heavily selected, so the detection rate there is not representative of all AUC patients, but it underscores why clinicians take the finding seriously enough to proceed with scoping.

When Cystoscopy Looks Normal but the Cytology Stays Abnormal

This scenario is more common than you might expect, and it is one of the trickiest situations in urology. A bladder that looks perfectly clean through the camera does not rule out cancer entirely. Small flat lesions, particularly carcinoma in situ, can be nearly invisible during standard white-light cystoscopy. Tumors can also lurk in the upper urinary tract or the prostatic urethra, areas the cytology can pick up but a standard bladder scope might miss.

When cytology remains persistently positive or atypical despite a normal-looking bladder, guidelines recommend evaluating the upper urinary tract and the urethra more thoroughly.¹¹ That might involve additional imaging, ureteroscopy (a scope that reaches the ureters and kidneys), or random biopsies of bladder tissue that appeared normal. Among patients with an initially indeterminate cytology result who underwent follow-up, about 41% eventually had a tumor identified, with the majority found at the second evaluation and most of those being high-grade.¹² Persistence of abnormal cytology, in other words, is a signal that deserves continued pursuit even when initial scoping is reassuring.

Urine Cytology’s Blind Spot for Low-Grade Tumors

One important limitation worth understanding is that urine cytology is much better at catching high-grade bladder cancer than low-grade disease. High-grade tumors shed cells that look dramatically abnormal, so pathologists can identify them with confidence. Low-grade papillary tumors, by contrast, shed cells that look relatively normal, and cytology has low sensitivity for detecting them.¹³ This means an AUC result that leads to a cancer diagnosis is more likely to be a high-grade tumor than a low-grade one.¹⁴ That is both good and bad news: good because low-grade tumors tend to grow slowly and have a better prognosis, bad because it means cytology alone can miss them.

How the Paris System Tightened the Definition

Before standardized criteria existed, the atypical category was something of a catch-all, and rates of AUC diagnoses varied wildly between institutions. The Paris System for Reporting Urinary Cytology, first introduced in 2016 and updated since, established specific nuclear features that should trigger an atypical classification. A cell needs to show a nuclear-to-cytoplasm ratio in a particular range along with at least one additional feature such as darker-than-normal nuclei or irregular nuclear borders.¹⁵

The practical effect has been to shrink the atypical bin. At one large institution, AUC rates dropped from about 19% of specimens before the Paris System to about 14% after implementation, while the proportion of AUC cases that went on to reveal high-grade cancer actually increased.¹⁶ Another study confirmed the same trend, showing the atypia diagnosis rate fell from 13% to 9% with tighter criteria.¹⁷ In other words, the Paris System is making AUC a more meaningful finding by weeding out borderline-normal cases, so if your report uses Paris System categories, the atypical label carries somewhat more weight than it might have a decade ago.

Molecular Tests That Help Clarify Gray-Zone Results

Because cytology alone cannot resolve every AUC case, several molecular and genetic add-on tests exist that try to push borderline results toward a firmer answer.

The most established is FISH testing (sold under the brand name UroVysion), which looks for specific chromosomal abnormalities associated with urothelial cancer. In patients with AUC cytology, FISH caught about 86% of true cancers, but its specificity was low, meaning it also flagged many people who did not have cancer.¹⁸ That high false-positive rate means a positive FISH result in someone with AUC cytology does not confirm cancer on its own; it needs to be interpreted alongside the rest of the clinical picture.¹⁹

A newer urine-based gene expression test called Cxbladder has shown promise for resolving atypical cytology. In one study, the test correctly identified all patients with atypical cytology who turned out to have urothelial carcinoma, including those who also had equivocal cystoscopy findings.⁵ Methylation-based urine tests have also entered clinical use for patients under surveillance after a prior bladder cancer diagnosis, though their role in first-time AUC patients is still being defined.

The Cost of Uncertainty

Gray-zone cytology results are not just stressful for patients; they carry real economic consequences. Every AUC finding triggers a chain of follow-up tests, and deciding which patients need an operating-room biopsy versus less invasive workup has meaningful cost implications. One economic analysis found that using FISH to guide biopsy decisions in patients with atypical cytology and equivocal cystoscopy saved roughly $1,740 per patient compared with biopsying everyone, while avoiding dozens of unnecessary procedures. Even when biopsies were performed in an office setting rather than an operating room, FISH-guided management still saved money.²⁰ As newer biomarkers prove their accuracy, the economic case for using them to triage AUC patients will likely strengthen.

Artificial Intelligence in Urine Cytology

The AUC category exists largely because human pathologists cannot always tell whether a borderline cell is reactive or neoplastic. Artificial intelligence may help close that gap. A multicenter study developed an AI model trained on urine cytology images that achieved sensitivity above 84% for detecting malignancy specifically within AUC cases, outperforming FISH in both retrospective and prospective validation.²¹ A separate study found that AI-assisted review reduced the rate of misclassifying AUC and higher-category cases as negative, and cut median review time by roughly two-thirds to three-quarters.²²

These tools are still in early clinical adoption, and no AI system has replaced pathologist judgment for urine cytology. But given that the atypical category is essentially a formalized expression of diagnostic uncertainty, it is exactly the niche where computational pattern recognition could add the most value. If the early results hold up in broader practice, AI-assisted cytology could eventually shrink the atypical category in much the same way the Paris System did, by resolving borderline cases that a human eye alone struggles to classify.

When Prior Cancer History Changes the Equation

If you are reading about AUC because you are being monitored after a previous bladder cancer diagnosis, the calculus is different from someone encountering the term for the first time after a routine test. Bladder cancer has one of the highest recurrence rates of any malignancy, and surveillance cytology is a standard part of long-term follow-up. In this setting, an AUC result sits against a backdrop of known prior disease, and clinicians tend to treat it with more urgency.

Patients receiving intravesical therapies like BCG or chemohyperthermia present a particular challenge. Their treatment deliberately inflames the bladder lining, producing reactive changes that can mimic cancer on cytology. One study of patients treated with these therapies found that when cases showing degenerated cells were excluded from the AUC group, the atypical diagnosis was significantly associated with a negative biopsy rather than cancer.²³ In other words, treatment-related cell changes were driving many of the atypical diagnoses, not recurrent tumors. Knowing this can spare some patients unnecessary biopsies, though it requires careful clinical judgment to decide which AUC results in a post-treatment bladder genuinely need tissue confirmation.

By contrast, a study focused on patients with no prior cancer history found that among those who went to biopsy after an AUC finding, about 65% had a malignancy confirmed, and a small number had a non-urothelial malignancy (a cancer originating from a different cell type).⁴ The takeaway is that prior history reshapes what AUC means for you personally, and your urologist will weigh it heavily when recommending next steps.