Serum sickness and serum-sickness-like reactions — formative SAQs

SAQ 1 (10 marks, 15 minutes)

Stem. A four-year-old boy is brought to the emergency department with a four-day history of a blotchy, moving rash over his trunk and limbs, low-grade fever and stiff, swollen knees. He finished a five-day course of cefaclor for an ear infection eight days ago. He looks miserable but not toxic, with normal observations. Outline your assessment, the diagnosis you would and would not make, the investigations that do and do not help, your stepwise management, and the avoidance decision you would defend. ^[8]

Model answer — SAQ 1

The history and presentation fit the textbook serum-sickness-like reaction: the triad of rash, fever and arthralgia arriving six to twenty-one days after a drug, with cefaclor the paradigmatic paediatric cause. The first step is to confirm the latency precisely and to exclude the dangerous mimics before settling on serum sickness. ^[8]

I would actively exclude anaphylaxis (the latency of over a week and the absence of airway or circulatory compromise argue against it), Stevens-Johnson syndrome or toxic epidermal necrolysis (no mucosal blistering, no skin pain, no target lesions with epidermal detachment) and DRESS (no eosinophilia or transaminitis, onset too early for the two-to-eight-week window). I would also consider a viral exanthem with reactive arthralgia, the commonest confounder in this age group, and a vasculitis such as Henoch-Schönlein purpura if palpable purpura is present. ^[13]

The diagnosis is clinical, built on the latency and the triad. The complement level is expected to be normal in a cefaclor SSLR, which distinguishes it from true serum sickness; I would check C3, C4 and CH50 if there is any doubt about a biologic exposure, but they are not required in a straightforward drug-induced case. I would not order a routine battery of specific IgE or skin-prick tests, because these are not IgE-mediated reactions and the results do not change the management or the avoidance stance. A urinalysis, blood pressure and basic bloods are reasonable to detect organ involvement and to grade severity. ^{[2] [5]}

My management follows the four-step ladder. Step one is to stop and document the cefaclor (already finished) precisely, including the indication, dose, latency and clinical features. Step two is symptom control: a non-sedating second-generation oral antihistamine at the local weight-based dose for the rash and itch, with a short course of oral corticosteroid reserved for severe arthralgia or a distressing rash, tapered over days. Analgesia for the joint pain is supportive. Step three is escalation only if organ involvement is present, which it is not in this case. ^{[2] [8]}

The avoidance decision is the part that most rewards careful thinking. A blanket lifelong beta-lactam label is not justified for a cefaclor SSLR. The modern evidence — the systematic review, the paediatric avoidance paper and the graded oral challenge data — supports an individualised approach: avoid cefaclor and closely related drugs for now, document the reaction, and arrange a specialist drug-allergy review with a graded oral challenge if a beta-lactam is clinically important in the future, because a coincidental viral exanthem is the common confounder and many labelled children tolerate the drug on challenge. I would also make clear to the family that this was serum sickness, not anaphylaxis or SJS, so the danger is not acute re-exposure in the same way. ^{[1] [3]}

Marking grid — SAQ 1

Common pitfalls — SAQ 1

• Applying a blanket lifelong beta-lactam label without considering a graded challenge.
• Ordering a routine specific IgE or skin-prick panel, which does not test the mechanism.
• Prolonging corticosteroid for a self-limiting SSLR.
• Failing to exclude SJS/TEN or DRESS before settling on serum sickness.