Limitations of the intradermal test for schistosomiasis mansoni: experience from epidemiologic studies in a Puerto Rican community.

The intradermal reaction with Schistosoma mansoni adult-worm antigen (35-40 microgram/ml nitrogen) was evaluated as an edpidemiologic tool in an endemic Puerto Rican community where the prevalence of S. mansoni infection was 36% and the geometric-mean egg count was 17.6 eggs/g. Subcutaneous injections of antigen were made in forearms, and stool specimens were examined for S. mansoni eggs by a formol-ether concentration method. Of 296 persons tested, 43% had positive intradermal reactions (greater than or equal to 1.0 cm 2 at least twice the area of the control wheal), compared to 48% positive stool examinations. However, sensitivity was low at 36% for children 14 yr old or less, and only 73% to 79% for adults. The test results were very specific for children (96%), but 32% of stool negative adults were positive. Mean wheal area was not directly related to intensity of infection as determined by egg counts in either children or adults, but did increase directly with age. Mean wheal areas were greater for males than females (both children and adults) at all intensities of infection. Because of unsatisfactory sensitivity and specificity the intradermal test may overestimate the prevalence of infection when rates are low, and underestimate prevalence of infection when rates are high. For its proper interpretation, complementary parasitologic data from stool surveys are required.