Serum amyloid A adds to anti-MDA5 when the lungs are the question

Design

retrospective single-centre cohort study with receiver operating characteristic and Cox analyses

Population

patients with dermatomyositis, including a rapidly progressive interstitial lung disease subgroup, plus healthy controls

Primary outcome

association of serum amyloid A with rapidly progressive interstitial lung disease, and with mortality

Effect

serum amyloid A 103.60 ± 18.02 mg/L in rapidly progressive disease; with anti-MDA5, area under the curve 0.950 (95% CI 0.906 to 0.995)

Rapidly progressive interstitial lung disease is what kills patients with dermatomyositis, and the dermatologist is often the first person to see them. A retrospective single-centre cohort measured serum amyloid A by scattering turbidimetry and asked whether it marks that risk.

Levels were far higher in dermatomyositis than in healthy controls (37.71 ± 6.93 versus 5.42 ± 0.30 mg/L; P<0.0001) and higher again in those with rapidly progressive disease (103.60 ± 18.02 mg/L). Raised serum amyloid A was independently associated with rapidly progressive interstitial lung disease, and combining it with anti-MDA5 antibody status gave the best discrimination, area under the curve 0.950 (95% CI 0.906 to 0.995). Survival was worse above a threshold of 21.98 mg/L (P<0.0001), and that threshold remained associated with mortality on multivariable analysis (hazard ratio 14.12, 95% CI 1.16 to 171.33; P=0.038).

What makes this worth attention is availability rather than novelty. Anti-MDA5 testing is send-away in most Indian centres and takes days to weeks; serum amyloid A is an acute-phase protein many hospital laboratories already run, with same-day turnaround. A high level in a new dermatomyositis patient is a reason to bring the chest imaging and the respiratory referral forward rather than booking them into the next available slot.

It is a triage signal, not a diagnosis. The finding does not replace high-resolution computed tomography or pulmonary function testing, and nothing here says treating the marker changes anything.

• Send serum amyloid A with the first bloods in new or flaring dermatomyositis, alongside the myositis antibody panel
• Treat a level above about 22 mg/L as a reason to expedite chest imaging rather than as a diagnosis
• Do not wait for the anti-MDA5 result before arranging respiratory assessment in a patient who looks unwell
• Record the classic cutaneous predictors too — ulceration, palmar papules, Gottron changes — the marker adds to them, not instead of them
• Repeat it rather than relying on a single value if the clinical picture is changing

Why it matters

It offers a same-day signal in a disease where the dangerous complication usually declares itself before the antibody result comes back.

Don't overread it

Retrospective, single-centre, and the threshold was derived in the same patients it was tested on — this is a triage aid awaiting validation, not a validated prognostic test.

The statistics, in plain English

The hazard ratio of 14.12 with limits from 1.16 to 171.33 is the number to read carefully: it crosses two orders of magnitude, which is what happens when few deaths are spread across a modest cohort. The direction is meaningful; the size is not estimable from this study. The area under the curve of 0.950 for serum amyloid A plus anti-MDA5 was derived in the same cohort that defined the 21.98 mg/L threshold, so it is an in-sample figure and will fall in any external validation.

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