Immunoglobulin replacement and antimicrobial prophylaxis — branching viva

Opening question

Take me through your framework when a child established on immunoglobulin replacement presents with breakthrough infection despite an apparently adequate trough. [2] [3]

Branch 1 — reassessment before a dose change

What are the four structured questions you must ask before simply raising the dose, and why is the trough an incomplete target for this child? [2] [3]

Branch 2 — choosing the route

What are the pharmacokinetic and practical advantages of switching this child to home subcutaneous immunoglobulin, and what is the evidence that supports it? How would you assess the family's capacity to deliver it? [5] [8]

Branch 3 — layering antimicrobial prophylaxis

Where does antimicrobial prophylaxis enter the pathway for this child, which agents would you consider, and what lung surveillance belongs in the plan? [6] [3]

Branch 4 — the contrasting case (the trap)

Now picture a child with specific antibody deficiency — normal immunoglobulin levels but a failed polysaccharide vaccine response. Why would you NOT start immunoglobulin here first, and what evidence justifies a trial of antibiotic prophylaxis instead? [6] [1]

Closing — the general rule

In one sentence, what is the single principle that decides how a child's immunoglobulin and prophylaxis regimen is built, and why does dosing to the outcome matter as much as choosing the route? [2] [8]