The tests & the diagnosis

Rheumatoid factor (RF) is the old marker: fairly sensitive but not specific (it also turns up in healthy older people, Sjögren's, and hepatitis C), so a positive RF supports but never confirms RA. Anti-CCP (ACPA) is the more useful test: similar sensitivity but far more specific (~95–98%), it predicts more erosive, aggressive disease, and it can appear years before symptoms. Being positive for both ('double-positive') points to more serious disease. Evidence ●●●●●

Seropositive RA (RF and/or anti-CCP positive) tends to be more aggressive and erosive. But ~20–30% of people have 'seronegative' RA — real disease with negative antibodies — which can be harder to diagnose and is not a milder condition; it needs the same intensive treatment. This is exactly why a rheumatologist, not a single blood test, makes the diagnosis. Evidence ●●●●○

ESR and CRP measure inflammation (disease activity, not diagnosis) — and they can be normal even in active RA. The formal 2010 ACR/EULAR criteria score joint involvement, antibodies, inflammation, and symptom duration (≥6 weeks), but they're a research classification that guides rather than replaces clinical judgement. Baseline bloods (blood count, liver, kidney) and, before biologics, screening for TB and hepatitis are standard. Evidence ●●●●○

Plain X-rays show erosions — but those are late damage, so a normal X-ray doesn't exclude early RA. Ultrasound with power Doppler shows active inflammation (and subclinical synovitis) in the clinic, quickly and without radiation; MRI is the most sensitive, catching bone-marrow oedema that predicts future erosion — the earliest sign of all. Imaging sensitivity runs X-ray < ultrasound < MRI. Evidence ●●●●○