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.

SAQ 2 (10 marks, 15 minutes)

Stem. A fourteen-year-old girl receiving rituximab for an autoimmune condition presents ten days after her second infusion with fever, an urticarial and annular rash, painful swollen knees and ankle joints, and tender cervical lymphadenopathy. Outline the diagnosis, the pivotal investigation, the mechanism, the severity assessment, and the treatment and avoidance plan. [11]

Model answer — SAQ 2

This is true immune-complex serum sickness, not an SSLR. The trigger is a foreign-protein biologic (rituximab), the indication is autoimmune (which carries a higher serum-sickness risk than haematological malignancy), and the latency of ten days fits the six-to-twenty-one-day signature. The mechanism is a Type III immune-complex hypersensitivity. [11] [5]

The pivotal investigation is the complement panel — C3, C4 and CH50 — which are expected to be low because the immune complexes activate and consume the classical complement pathway. I would also check urinalysis (for proteinuria), renal function, liver function and a full blood count, looking for the organ involvement that escalates severity, and I would draw the complement during the active episode when consumption is most visible. The lymphadenopathy and the autoimmune context further support true serum sickness rather than an SSLR. [5] [11]

The mechanism explains the management. In true serum sickness, the foreign-protein antigen combines with newly formed antibody over the latent period, the resulting immune complexes deposit in vessel walls, synovium and renal tissue, complement is consumed (which is why C3, C4 and CH50 fall), and neutrophil-mediated inflammation produces the triad. This is fundamentally different from an SSLR, which reproduces the triad with a normal complement and no immune complexes. [5]

For severity, I assess for organ involvement — nephritis (proteinuria, haematuria, rising creatinine), serositis, and the degree of synovitis and systemic illness. For significant symptoms or any organ involvement, I give systemic (often intravenous) corticosteroid and admit under the team managing the autoimmune disease, coordinating re-treatment with an alternative immunosuppressive agent. Adrenaline is not the treatment, because this is not IgE-mediated, but standard paediatric supportive care applies if the child is unwell. [11]

The avoidance decision is unequivocal: rituximab should not be re-challenged, because recurrence of immune-complex serum sickness can be faster and more severe on repeat exposure. This is a different decision from the individualised approach taken for a drug-induced SSLR. I would document the reaction precisely, communicate the non-re-challenge status to the prescribing subspecialty, arrange a medical-alert record, and counsel the family about the distinction between this and an acute allergic reaction. [11] [5]

Marking grid — SAQ 2

Common pitfalls — SAQ 2

• Treating true biologic serum sickness as an SSLR and applying a permissive avoidance stance.
• Re-challenging rituximab after an immune-complex serum-sickness episode.
• Omitting the complement panel and so missing the discriminator.
• Forgetting to assess for and manage organ involvement.