People often arrive with a folder of blood tests and a single question: what is wrong with me? The tests are useful, but they answer a narrower question than most people expect. Here is what each of the common ones actually contributes.
ESR and CRP
Both measure inflammation somewhere in the body. Neither says where, and neither says why. An infection, a recent injury, obesity, pregnancy and anaemia all move them.
The important limitation is the other direction. Active inflammatory arthritis can occur with a completely normal ESR and CRP. This is common in psoriatic arthritis and in axial spondyloarthritis. A normal inflammatory marker is not permission to stop looking.
Where they earn their place is in following disease that is already diagnosed: a falling CRP on treatment is good evidence that the treatment is working.
Rheumatoid factor
Rheumatoid factor is positive in about seventy per cent of people with rheumatoid arthritis. It is also positive in a proportion of healthy people, and the proportion increases with age. It is positive in chronic hepatitis C, in Sjogren's syndrome, and in some chronic infections.
So a positive rheumatoid factor in someone with no joint symptoms usually means nothing at all, and a negative one in someone with swollen hand joints does not exclude rheumatoid arthritis. Around thirty per cent of people with rheumatoid arthritis are rheumatoid factor negative.
Anti-CCP
Anti-CCP is the more useful of the two. It is far more specific for rheumatoid arthritis: a positive result in someone with joint symptoms is strong evidence. It also carries prognostic weight, since anti-CCP positive disease tends to be more erosive and is treated more assertively from the outset.
It can be positive years before symptoms begin, which is occasionally how a diagnosis is anticipated rather than waited for.
Uric acid
The commonest misunderstanding in the whole of rheumatology sits here. Uric acid is often normal during an acute gout attack, because the level falls as crystals precipitate into the joint. A normal level in the middle of an attack does not rule out gout.
Equally, a raised uric acid level in someone with no symptoms is not gout and, on its own, is usually not treated. The diagnosis is made on the clinical picture and, where there is doubt, by identifying crystals in fluid drawn from the joint.
Uric acid is genuinely useful once treatment has begun: it is the number that is tracked to a target, and reaching that target is what stops attacks permanently.
HLA-B27
This is a genetic marker, not a disease test. It is present in a substantial minority of the healthy population, and most people who carry it never develop anything. It raises or lowers the probability of axial spondyloarthritis in someone who already has inflammatory back pain — nothing more.
Ordering it in someone with ordinary mechanical back pain generates anxiety and no information.
ANA and the antibody panels
A positive ANA is common in healthy people and needs interpretation rather than alarm. Broad antibody panels ordered without a clinical question produce incidental positives that then have to be explained away. The specific antibodies — anti-dsDNA, anti-Ro, anti-Scl-70 and the rest — are worth ordering when the clinical picture already suggests what you are looking for.
What actually makes the diagnosis
The history and the examination. How the symptoms began, what pattern they follow, what makes them better and worse, which joints are involved and whether they are truly swollen. Blood tests, imaging and, where appropriate, fluid drawn from a joint then confirm or refute what the history suggested.
If you are bringing results to a consultation, bring all of them, including the old ones — a value that has changed over two years is often more informative than any single reading.
This article is general information and does not replace a consultation.