Question

A company has developed a new dipstick test for urinary tract infection (UTI). The test shows either “positive” or “negative” (there are no equivocal results). You wish to test this in your A&E, where, on average, 15% of your patients under 1 year of age with fever have a true UTI, as diagnosed by a positive urine culture (> 10⁵ cfu/ml) when obtained under sterile conditions (e.g. suprapubic aspirate or catheter-obtained urine).

Over the next 6 months, you evaluate the dipstick on 100 patients less than 1 year of age presenting with fever, and obtain the following results:

		N
Urine Culture +	Dipstick +	10
Urine Culture +	Dipstick -	2
Urine Culture -	Dipstick +	11
Urine Culture -	Dipstick -	77

Your colleague, Dr Bayes, in a different A&E decides, on the basis of your evaluation, to adopt the dipstick test. Her A&E has a different catchment area to yours, and has an estimated UTI rate of 8% in febrile infants.

would you use the test in either A&E? What factors may help you decide?

Answer 1

SOLUTION:

For Dr.Bayes lab:

Now,

In my lab : P(actual+) = 0.05 ; after carrying out similar calculations,

as done above with Dr. Bayes lab, we get for my lab,

P(actual+ /dipstick+) = 0.5405

So, in my lab, this probability > Mr. Bayes lab probability.

This is justfied because in my lab there is higher prevalence i.e.

15 % of prevalence than 8% in Mr. Bayes lab.