A 6-month-old infant from rural Maharashtra presents to the immunisation clinic. The mother reports that the child received BCG at birth and first dose of DPT at 6 weeks, but missed the second dose of DPT at 10 weeks due to illness. On examination, the child appears healthy with no signs of tuberculosis or other infections. The health worker wishes to confirm the immunological status of the child before administering the catch-up dose. Which investigation is most appropriate to assess the child's response to the DPT vaccine already administered?