The Evidence Behind Your Diagnosis Is Weaker Than Anyone Has Told You

The colony count threshold used to diagnose UTI from a urine culture has been in place since 1957. It came from a study of 88 pregnant women with kidney infections. It was never designed to diagnose the kind of UTI that most people reading this actually have.

That is where this episode starts to get genuinely important.

A Threshold Built on the Wrong Population

Dr Catriona Anderson explains this with the kind of clarity that most clinical appointments never get near. The Kass criteria (still used as the basis for determining whether a culture is positive or negative) were challenged in 1980 by Stamm et al., who showed the threshold was missing a significant proportion of women with genuine symptomatic infection. A call to improve things followed. It was largely ignored.

“When it says no significant growth, does that mean there's no growth at all? Because from what I've read, that means there could be growth, but actually it's just not reaching the threshold." ~ Dr Cat

This is not a niche academic argument. For people with chronic UTI who keep being told their tests are negative, understanding exactly what those tests are and are not measuring changes the entire landscape of what they can ask for.

The Mouse Model Problem

The evidence base for chronic UTI treatment has another layer of fragility that this episode surfaces. Professor Jennifer Rohn's research highlights that a significant proportion of antibiotic studies in bladder conditions have been conducted on mice – animals that don't naturally develop UTIs and have to be artificially induced to get them. The physiological differences between rodent and human bladders are meaningful and what works in a forced infection model in a laboratory does not necessarily translate to a slow-growing chronic infection in a real person.

The Three-Day Antibiotic Course

Medical misogyny shows up most clearly in the antibiotic duration disparity. Women with uncomplicated UTI receive three days. Men receive seven. This is said to reflect anatomical differences but the meta-analysis data that informed the three-day recommendation for women actually showed that five-day courses gave better bacterial clearance. The shorter course was adopted anyway, partly on antimicrobial stewardship grounds, partly on the basis that most women felt symptomatically better at three days.

“You can do a huge meta-analysis, but a meta-analysis of rubbish still ends up being rubbish." ~ Dr Cat

For people with chronic UTI who know they don't respond to three days, this episode makes clear that asking for longer is not only reasonable, the December 2024 NICE guidelines explicitly permit it.

Listen to the full conversation with Dr Catriona Anderson and Neha now.

Resources can be found here.