A pilot study was undertaken to assess the validity of two new tests for predicting the immune response of Toronto schoolchildren with no acceptable evidence of prior administration of
diphtheria or
tetanus toxoid to a routine booster injection of
diphtheria and
tetanus (DT)
toxoid. The tests, an inexpensive
enzyme-linked
immunosorbent assay (ELISA) fingerprick test for
tetanus antibodies and a modification of the Schick skin test for susceptibility to
diphtheria, were administered before the booster injection. One week later the ELISA test was repeated and the result of the modified Schick test read. On both occasions a
diphtheria microneutralization assay was done for "gold standard" evidence of prior exposure to
diphtheria toxoid or toxin. The results were used to determine the sensitivity and specificity of a single prebooster
tetanus ELISA test or a modified Schick test for predicting which children with no records could be safely protected with only one DT booster dose instead of the primary series of three or four doses usually given to such children. Only 6 of the 34 subjects (18%) were totally without prior exposure to
tetanus toxoid. Two of the six (6% of 33 subjects) appeared to mount a primary immune response to
diphtheria toxoid as well. An initial ELISA titre of 0.01 IU/ml or lower correctly identified all six children needing a full series of
tetanus toxoid (sensitivity for a primary immune response 100%) and falsely identified only 3 of 28 immune children as needing the series (specificity for immunity 89.3%). The modified Schick test appeared to have even greater accuracy for identifying children needing a full series of
diphtheria toxoid. However, its use, entailing the costs of an extra nurse visit, would have prevented only seven more children from receiving an unnecessary full series of
diphtheria toxoid than use of the baseline
tetanus ELISA test alone.