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Editor,—We read with interest the letter by Thayyil-Sudhan and Gupta reporting their study on the role of dipsticks in the detection of urinary tract infection in children.1We believe that this is a very important subject and wish to comment on the report and their conclusions in the light of our published study.2
We note that as 188 urine samples were not sent for culture, it is not possible to determine the number of true and false negative dipstick tests (if any). Without these data, calculation of sensitivity and specificity of dipstick testing becomes impossible.3Because of the above we believe that the data presented are skewed secondary to a flawed experimental design. Consequently, the statement of the authors that urinary tract infection in children cannot be excluded by a negative nitrite or leukocyte esterase reaction is difficult to justify. Furthermore, there is no information to indicate whether children who were being treated with antibiotics at or immediately before admission were included in the study. If this is the case, the possibility of false negative culture results cannot be excluded and this will add further bias to the results. No data are provided for the number of infants included in the study. It has been reported that negative dipstick tests have a higher false negative rate in infants or in cases of urinary frequency because decreased bladder incubation time diminishes in vivo bacterial multiplication.4 We are not told about the percentage of the samples, which were collected by pads, as compared to midstream specimens as this may further add to the inaccuracy of the culture results.
In our prospective study of 325 children in whom urinary tract infection was a clinical possibility, all urine was sent for laboratory examination.2 The laboratory was unaware of the results of the dipstick tests until the end of the study. Analysis of our data showed that the combination of negative dipstick tests for nitrite and leucocyte esterase gave a negative predictive value for urinary tract infection of 96.9%, with a specificity of 98.7%. The figures for infants were 96.7% and 99.2%, respectively. A positive nitrite and/or leucocyte esterase had a positive predictive value of 60% and a sensitivity of 54.6%, compared with 50% and 20.0% respectively in infants. In our series we found that there were four false negative and six false positive nitrite tests.
The dipstick tests are most likely to be useful as a screening test to exclude urinary tract infection in children but may be less suitable for infants. They should not be used to diagnose urinary tract infection. We therefore disagree with Thayyil-Sudan and Gupta in their view that if nitrites are positive, starting empirical treatment for urinary tract infection seems to be reasonable until cultures are reported.
Dr Thayyil-Sudhan and Dr Gupta comment:
Our study involved a selected group of children who were at an increased risk of having urinary tract infection.1-1 The inclusion criteria were the presence of any of the following: firstly, clinical suspicion of urinary tract infection; secondly, history of previous urinary tract infections or renal anomalies; thirdly, children needing antibiotics (urine culture was sent before starting antibiotics); and finally, any of the dipstick tests (nitrites, protein, leukocyte esterase, or blood) being abnormal.
Out of the 500 children admitted to the hospital during the study period, only 312 met the above criteria and were included in the study. Urine culture was done for all these children, which reflects the local practice at our hospital of sending urine for culture. We wanted to see if a change in practice to urine culture being done only if nitrites or leukocyte esterase were positive would be effective in reducing the number of urine cultures.
The inclusion criteria for Sharief and colleague's study1-2was a clinical suspicion of urinary tract infection, when urine cultures were sent and dipstick testing was done. We found that urinary tract infection could easily be missed if urine culture is undertaken only if nitrites or leukocyte esterase are positive. Surprisingly, the results of both our study and theirs are similar: sensitivity was 34.4% v 20.0% and specificity was 90.7%v 99.2% in our study and Sharief's study respectively. Negative predictive value was 92.4% in our study and 96.7% in Sharief's study. Only the interpretation of the results is different.
A test with such a low sensitivity cannot be recommended as a screening test to exclude urinary tract infection. Urinary tract infection may result in irreversible renal damage in infants and therefore most care should be given to the detection of this infection in this age group. Unfortunately, this is the age group where sensitivity of dipstick testing is the lowest (20%). I agree with Sharief and colleague's study3 that because of its high negative predictive value, dipstick testing may have some role as a screening test for urinary tract infection in situations where the incidence is very low. Positive nitrites have a high specificity for urinary tract infections, which was the basis of our suggestion that if nitrites are positive, especially in a febrile infant, empirical treatment with antibiotics may be considered until the result of urine culture is obtained. However, it should not be the whole criterion for diagnosing this infection.