Palpable Purpura: Workup
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Direct answer
Palpable purpura means inflamed vessels, not failed platelets — a small-vessel leukocytoclastic vasculitis you can feel with a fingertip, demanding a cause-hunt rather than a haematinic prescription. The search sweeps five shelves: drugs (antibiotics, NSAIDs, diuretics), infections (streptococcal, hepatitis B and C, endocarditis, meningococcaemia), connective tissue disease (lupus, rheumatoid arthritis), ANCA-associated vasculitides (granulomatosis with polyangiitis, microscopic polyangiitis) and immune-complex syndromes such as IgA vasculitis and cryoglobulinaemia. Biopsy is the pivot — histology from a 24-48 hour lesion for neutrophils, fibrinoid necrosis and nuclear dust, direct immunofluorescence from a lesion under 24 hours where IgA deposition defines Henoch-Schonlein purpura — alongside urinalysis, ANCA, complement and hepatitis serology.
What you must remember
- Definition logic: non-blanching, raised purple lesions = vessel-centred inflammation; flat purpura with normal vessels points instead to thrombocytopenia, senile purpura or coagulopathy — the first fork in every question.
- Biopsy timing (the pearl): histology from a 24-48 hour lesion for leukocytoclasia, fibrinoid necrosis and red-cell extravasation; direct immunofluorescence from a lesion under 24 hours — older lesions lose their immunoglobulin and waste the test.
- IgA vasculitis (Henoch-Schonlein): paediatric-age palpable purpura on buttocks and extensor legs plus arthritis, abdominal pain and nephritis; IgA-dominant deposits on DIF; ASO and streptococcal serology where relevant.
- Core bloods: full blood count, ESR, CRP, urinalysis for protein and blood, renal and liver function — urinalysis being the cheapest renal-vasculitis detector on the list.
- Immunology set: ANA, C3 and C4 (low in cryoglobulinaemia and lupus), ANCA with PR3 and MPO antibodies, cryoglobulins sent warm, rheumatoid factor (often positive with mixed cryoglobulinaemia).
- Infection set: hepatitis B surface antigen, anti-HCV, blood cultures if febrile, ASO titre or anti-DNase B in the post-pharyngitis child.
- Retiform variant: geometric, branch-like purpura signals thrombotic or embolic occlusion — warfarin necrosis, DIC, cryoglobulinaemia, cholesterol emboli — with biopsy showing bland thrombi rather than neutrophils.
- Indian angles: dengue and other thrombocytopenic fevers give flat purpura (a favourite trap); erythema nodosum leprosum gives tender nodules on shins that mimic vasculitis; tuberculosis and post-streptococcal states pepper the cause lists.
Working through a case from lesion to label
A 34-year-old man arrives with crops of tender purple papules on both shins and ankles over ten days, some blistering, after a sore throat two weeks prior; he takes no regular drugs. Step one: confirm the lesion is palpable and non-blanching — vasculitis, not a platelet problem — though a baseline platelet count still belongs in the panel. Step two: biopsy pair — punch from a 36-hour lesion for histology, and a fresh 12-hour lesion snap-frozen for DIF. Step three: the panel — urinalysis traces blood (kidney involvement, the finding that changes prognosis), ANCA negative, complements normal, ANA negative, ASO raised, hepatitis serology negative. The constellation — post-streptococcal palpable purpura, arthralgia, microscopic haematuria, DIF showing IgA deposition — completes IgA vasculitis. Management is supportive with NSAIDs for joints, close blood-pressure and urinalysis surveillance, and nephrology referral because renal involvement decides long-term outcome. Had ANCA returned positive with cavitating lung lesions, the same legs would have redirected the patient to granulomatosis with polyangiitis and cyclophosphamide — identical skin, different life.
Where candidates slip
The first slip is panel-only thinking: diagnosing "vasculitis" by bloods without biopsying, then being unable to answer the DIF question — IgA deposits define IgA vasculitis, and the specimen must be fresh, because immunoglobulin degrades in lesions older than a day. The second is forgetting that the urinalysis is part of the skin workup: finding nephritis upgrades disposition and prognosis regardless of the cause. The third is the retiform trap — geometric purpura is thrombotic (warfarin necrosis, DIC, cholesterol emboli), and treating it as inflammation misses a haematological or vascular emergency. Finally, in Indian wards, the febrile child with purpura is as often dengue with thrombocytopenia — flat, non-palpable lesions — as genuine vasculitis; palpability is the discriminator you are being tested on.
Frequently asked questions
What does palpability of purpura signify?
Inflammation of vessel walls with cellular infiltrate raising the lesion, indicating vasculitis rather than platelet or coagulation-failure bleeding.
Why must the immunofluorescence biopsy come from a fresh lesion?
Immunoglobulin and complement degrade within about 24 hours, so an older lesion yields false-negative results in IgA vasculitis and immune-complex disease.
Which investigations screen for renal vasculitis?
Urinalysis for protein and blood, serum creatinine, and blood pressure measurement — simple tests that detect the complication that matters most.
What does a retiform pattern of purpura suggest?
Thrombotic or embolic occlusion — warfarin necrosis, DIC, cryoglobulinaemia or cholesterol emboli — with bland thrombi rather than neutrophilic vasculitis on histology.
Which infection serologies belong in the palpable purpura panel?
Hepatitis B surface antigen, anti-HCV, ASO or anti-DNase B for streptococcus, and blood cultures when endocarditis or meningococcaemia are plausible.