The usefulness of dipstick testing when predicting UTI

Dipstick tests are routinely used to diagnose UTI, but the evidence is mixed, says Professor Paul Little

Acute UTI is one of the most common acute bacterial infections in women.

In clinical practice the universal use of MSU testing to make a diagnosis is not likely
to be a cost-effective use of resources. MSU testing should probably be reserved for men, children, pregnant women and those who have complicated infections.

Antibiotic resistance
For the majority of those presenting (that is, non-pregnant adult women with uncomplicated infection) empiric antibiotic treatment has been advocated.

However, the potential problem with universal antibiotic use is the growing problem of antibiotic resistance.

Because antibiotic resistance occurs in more than 20 per cent of laboratory specimens, the opportunity to make a more accurate diagnosis of UTI using urinary dipstick tests should be strongly considered.

Dipsticks are perhaps the most widely used simple near-patient tests in primary care. Many GPs use them as a first-line investigation either to rule in or rule out UTI.

Evidence considered
The question then is what is the evidence that we can use dipsticks for better diagnosis and targeting antibiotics?

A further question that should be asked is how much better are dipsticks when
compared with simply using clinical acumen?

Summary data are available from studies that assessed nitrite and leucocyte esterase separately.

However, it would be useful and helpful to know which combination of the different dipstick results, including protein and blood in addition to nitrite and leucocyte esterase, are the most useful when making a diagnosis.

One systematic review suggested that the evidence base for dipstick use in primary care is poor.

This may be because of the lack of studies and likelihood of ‘spectrum bias’, that is, a change in diagnostic accuracy when extrapolating from studies based in other settings.

Other evidence from primary care has either not assessed the independent value of dipstick results (resulting in overcomplicated clinical decision rules) and/or had low power, and/or had mixed clinical and dipstick variables.

A recent study assessed the independent predictive value of dipstick results when compared with laboratory results among women presenting with suspected UTI in primary care, of whom 60 per cent had confirmed UTI.

Having either nitrite or both leucocytes and blood (haemolysed trace or greater) on testing the urine detected most patients with UTI (sensitivity 77 per cent), was reasonably predictive (positive predictive value 81 per cent) but had a moderately low negative predictive value (65 per cent).

The negative predictive value only increased to 73 per cent when all three dipstick results were negative.

This means that 27 per cent of the time a woman with dipstick results which are all negative will in fact have UTI because the results are false.

Therefore this shows that dipsticks cannot sensibly be used to rule out infections and are not entirely accurate.

Clinical symptoms
An important question that clinicians should ask is whether the pattern of presentation in suspected cases of UTI can be used to improve the diagnostic accuracy.

In the same study as discussed above, a ‘clinical rule,’ based on having two of urine cloudiness, offensive smell,  moderately severe dysuria and moderately severe nocturia, was less sensitive (65 per cent) than dipsticks in predicting laboratory confirmed UTI.

The negative predictive value was 71 per cent for none of the four clinical features, and the positive predictive value was 84 per cent for three or more clinical features.

Therefore, this means that although the pattern of clinical symptoms and signs can help in confirming the diagnosis they are not very helpful in ruling out UTI.

These results also indicate that dipsticks perform slightly better than clinical information alone, but the results also indicate that they are not that much better.

Self-treatment
There is still debate about the appropriateness of antibiotics for UTI.

Most women with symptoms of cystitis treat themselves conservatively without needing antibiotics.

The placebo groups of randomised controlled trials also suggest that women not treated with antibiotics mostly get better (albeit more slowly) with low complication and recurrence rates.

This may therefore mean that missing 27 per cent of those with infection may not be absolutely crucial.

The main issue is that UTI is a distressing, uncomfortable condition and that it significantly interferes with ordinary daily life.

Women and their doctors may not feel reassured that they do not need antibiotics based on a potentially false negative dipstick result, which could result in patients spending a couple more days in significant discomfort.

What should GPs do?
The current evidence base supports empiric treatment with a short course of antibiotics irrespective of dipstick results.

But it also supports treating those with positive nitrite or positive blood and leucocytes based on dipstick results.

Women with negative dipstick results should then be asked to return if their symptoms are not settling or you could consider giving them a delayed antibiotic prescription.

What do I do with patients? I negotiate with women as to which strategy they are most comfortable with.

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