Near-patient testing for infection using urinalysis and immuno-chromatography strips

Urinary tract infections require costly confirmatory tests such as a urine culture to establish the diagnosis. Elimination of the culture step would save resources; diagnosis and treatment could begin in hours rather than days. We tested a new dip-and-read strip that uses immuno-chromatography (IC) to detect infectious agents in urine. We used a goat-derived polyclonal antibody with reactivity to the cell-wall proteins of Escherichia coli (E. coli). Fluorescein linked to the anti-E. coli antibody served to trap the bacteria on a strip coated with an anti-fluorescein mouse antibody. Blue latex particles were linked to anti-E. coli antibodies by standard methods and were used for detection of E. coli. We found that the combination of leukocyte esterase and nitrite dipsticks gave negative predictive values of 93% for culture-negative urines, i.e., there were very few false-negative results. Using the same dipsticks on culture-positive specimens, the positive predictive values were unacceptably low; we obtained too many false-positive values. By contrast, the IC strips gave negative predictive values of 89%. The major advantage of the IC strips is that the positive predictive values were higher, i.e., there were fewer false-positive results. The combined use of both IC strips and urinalysis dipsticks offers the best strategy for diagnosing infection with dipsticks. The IC strip test could reduce the necessity of a urine culture in patients with suspected infections and provide rapid point-of-care testing.