Factors affecting the reliability of screening tests for bacteriuria. I. Nitrite test (Urnitest), Uriglox and dip-slide (Inculator)

Using urine nitrite sticks to test for urinary tract infection in children aged < 2 years: a meta-analysis

BACKGROUND

This study aimed to determine whether nitrite sticks are as sensitive at detecting urinary tract infection (UTI) in children <2 years as they are in older children.

METHODS

I reanalysed data on using nitrite sticks to detect UTIs for children aged either < 2 or 2-18 years. For sensitivity, evidence of a UTI was defined as level 1 when a single uropathogen grew ≥ 10⁵ colony forming units/ml (cfu/ml) in two urine samples, level 2 when just one sample was cultured or a threshold of < 10⁵ cfu/ml was used, and level 3 if mixed growths or Staphylococcus albus was considered to be positive. For specificity, children were defined as uninfected if they had 1 sterile urine culture. I also reanalysed our previously published data by age.

RESULTS

The sensitivity was lower for children aged < 2 years (11 studies, 1321 subjects) than for older children (9 studies, 295 subjects), whether the level-1 values or all the studies were analysed (Fisher's exact test, p < 0.0001 for both). The level-1 sensitivities were 0.23 in the infants and 0.81 among older children (odds ratio = 0.07, 95% confidence interval 0.03-0.18). The specificity was very high in infants (10 studies, 1783 cases) and older children (7 studies, 5952 cases), at 0.990 and 0.996.

CONCLUSIONS

Nitrite sticks only have a 23% sensitivity in children aged < 2 years, so cannot reliably rule out UTIs. A positive nitrite stick test is about 99% likely to indicate a UTI in children of any age.

Evaluation of rapid methods for the detection of bacteriuria (screening) in primary health care

The diagnostic performance of six methods for bacteriuria testing has been studied in 781 urine specimens obtained in primary health care, using conventional culture as reference method. The cut-off limits for classification of test results into positive and negative have been optimized with respect to diagnostic performance in primary health care. With optimized tests the following diagnostic efficiencies were obtained: Bacterial ATP, 0.94; Bacterial count in sediment, 0.93; Nitrite test, 0.92; Dipslide test, 0.92; White cell count in sediment; 0.87; Goffulocyte esterase test; 0.83. The diagnostic performance was also studied for all combinations of two tests. The highest diagnostic efficiency (0.96) was obtained by combining the ATP and dipslide tests. High diagnostic efficiencies can be obtained by a rapid primary test, using other tests for follow-up testing of specimens with intermediate or uninterpretable primary results. The most promising results were obtained by using ATP as the primary test, with follow-up testing of specimens with 3-25 nmol/l of ATP (12 per cent of the specimens). Follow-up testing by conventional culture resulted in overall diagnostic efficiency of 0.98. By performing the nitrite test on specimens with intermediary ATP-results, 81 per cent of the patients with UTI can be classified without culture. Only patients with intermediary ATP and negative nitrite results (10 per cent of the total number) will have to wait for final diagnosis based on conventional culture. Some alternative strategies to combine available methods are discussed in detail. Major advantages of the ATP test are that the test can be performed while the patients are waiting; it provides a numerical and objective result, and, in contrast to culture, it is not influenced by adhesion of bacteria to somatic cells.

Firefly luciferase ATP assay as a screening method for bacteriuria

A rapid (15 min) test for bacteriuria based on firefly luciferase analysis of bacterial ATP has been evaluated in 2,018 clinical urine specimens. The test procedure involves removal of nonbacterial ATP by treatment of urine with Triton X-100 and apyrase, extraction of bacterial ATP by boiling, and bioluminescent analysis of bacterial ATP by firefly luciferase, using a luminometer. For comparison, the widely used nitrite test was included in the study as an example of an alternative rapid chemical test. The test was set up to distinguish between specimens yielding greater than 10(5) CFU/ml and specimens yielding less than 10(5) CFU/ml. A level of 13.5 nM ATP was chosen to define the limit between negative and positive results. At this discriminatory level, 92% of specimens yielding greater than 10(5) CFU/ml and 88% of specimens yielding less than 10(5) CFU/ml were correctly classified with the luciferase method, whereas corresponding figures for the nitrite test were 55 and 99%, respectively. Of the 12% false luciferase positives, 20% were shown to contain greater than 10(5) CFU/ml on prolonged incubation, thus reducing the false-positive rate to 10%. Of the 8% false luciferase negatives, 65% had low levels of CFU in the range of 10(5) to 10(6).

Screening for bacteriuria of schoolchildren by the nitrite reaction

The author is conducting systematic screening surveys for bacteriuria among the pupils in the primary schools (6-14 years) of a provincial town and environments. He applies the nitrite reaction for the detection of bacteriuria. Over the past 5 years 16,621 children, 7982 girls and 8639 boys, were screened, part of them repeatedly. Four bacteriuric cases were diagnosed among the boys and 130 among the girls. In a single screening the prevalence rate of bacteriuria proved to be 1.23% among the girls. In repeated screenings the incidence rate was 0.44% on the average per annum. Fifty-six of the 130 bacteriuric girls were subjected to general examination; radiological abnormalities were found in 26 of them. The author considers the systematic screening of schoolchildren as reasonable. The nitrite reaction is the most appropriate for mass screening, since it is simple, immediate, inexpensive, specific and sufficiently sensitive.