Letter: Nitroblue-tetrazolium test in cystic fibrosis.

It has been claimed that the nitroblue tetrazolium (N.B.T.) test is of 
value in the surveillance of children with cystic fibrosis, a positive 
test giving evidence of active bacterial infection which may not be 
apparent clinically and not made evident by such investigations as the 
white-blood-cell count, sputum culture, and chest X-ray. The test was therefore 
included in the routine assessment of patients attending this clinic. Of 
the total 269 N.B.T. tests, 110 were positive. In only 57 
of these cases was there other evidence of infection; these 57 
were treated with antibiotics. The remaining 53 were left untreated. Of 
these, 41 remained completely free of infection by standard criteria at 
subsequent review. In 12 cases, however, the patient developed obvious infection 
before the next clinic visit. It could be argued that in 
those cases the N.B.T. test had given early evidence of infection 
not otherwise apparent. This is by no means certain, since the 
onset of infection could have been at any time during the 
four-week interval. In our experience, the N.B.T. has been of little 
help in the management of cystic fibrosis. There were in this 
series 27% false-negatives and at least 37% false-positives, some of which 
had very high counts. It is possible, however, that in 12 
(i.e., 4%) of the tests carried out, the N.B.T. gave early 
indication of infection not otherwise suspected. 