Absence of symptoms and negative dipstick results do not rule out infection in patients with suspected UTI, a primary care study has found.

The researchers aimed to identify which clinical features and dipstick variables independently predict laboratory diagnosis of UTI, and to validate clinical decision rules based on these independent predictors.

A total of 434 women with suspected UTI were recruited to the study from practices in the south of England. They were asked to rate the presence of symptoms and their severity. A midstream urine specimen was examined for cloudiness to the naked eye, and offensive odour, before urine dipstick testing was carried out.

On dipstick testing, a positive test for nitrite was found to be the best predictor of UTI, followed by blood and then leucocytes. The rule derived from previous studies was that a positive nitrite result, or positive result for both blood and leucocytes, was considered to be indicative of a UTI.

This study found that the rule had a sensitivity of 75%, but a specificity of only 66%. This meant a positive predictive value (i.e. positive test correctly identifying UTI) of 81%, and a negative predictive value (i.e. negative result correctly ruling out UTI) of only 57% - this improved to 76% if all three variables tested were negative. Thus, even when all these dipstick results are negative, nearly a quarter of women presenting with suspected UTI actually have UTI.

With respect to the clinical features, only the presence of dysuria, nocturia or cloudy urine were reliable independent predictors of UTI. Offensive smelling urine was not found to be a useful predictor. However, while patients with dysuria, cloudy urine and nocturia are very likely to have UTI, even when all these features are absent, 33% of patients presenting with suspected uncomplicated UTI turn out to have a UTI on laboratory testing.

Thus, both dipstick parameters and clinical indicators are useful but have serious limitations. Negative predictive values are a particular problem.

GPs need to be aware that in a patient with suspected UTI, absence of classic symptoms or negative dipstick results do not reliably rule out the presence of infection. Thus, alternative strategies are required for these patients, whether arranging clinical review if symptoms persist or the use of delayed prescriptions.

Dr Jonathan Rees