Primary immunodeficiency: warning signs and diagnostic approach — branching viva

Opening question

Examiner: Take me through this infant. What is your reading of the presentation, and what is your frame? [1]

Candidate: This infant has the signature of a combined immunodeficiency. He has failure to thrive, persistent oral candidiasis beyond infancy, two pneumonias with one needing intravenous antibiotics, and a lymphocyte count that — although it appears normal at an adult threshold — is low for age, because infants normally have high lymphocyte counts. He also has the family-history warning sign: a male cousin who died in infancy from an unexplained infection. My frame is that this is severe combined immunodeficiency until proven otherwise, which is a time-critical diagnosis, and my response is a same-day immunology referral with flow cytometry and a pathway to transplantation. [1]

Examiner: Why is a lymphocyte count of 1.8 × 10⁹/L abnormal in a seven-month-old when it would be normal in an adult? [5]

Candidate: Because lymphocyte counts are age-specific. Infants normally have an absolute lymphocyte count of five to seven × 10⁹/L or higher, so a count of 1.8 is in fact lymphopenic for age. This is one of the commonest reasons SCID is missed — the count is read against an adult threshold rather than an age-appropriate one, and the warning is lost. [5]