A 6-month-old infant presents to the immunisation clinic for routine vaccination. The mother reports that the child received BCG at birth and DPT dose 1 at 6 weeks of age. On examination, the child is afebrile and clinically well. The healthcare worker suspects a documentation error regarding previous doses. Which investigation is most appropriate to confirm the immune status of this child against diphtheria, tetanus, and pertussis before proceeding with the next scheduled dose?