A big toe that turns hot, red and painful overnight is the picture many people have of gout. Behind it is a substance most of us never think about: uric acid. Measuring it is simple, but reading it well takes a little context. This guide explains what uric acid is, how the uric acid blood test is used around gout, why the result can mislead during a flare and how it helps follow treatment over time.
What uric acid is
LML describes uric acid as a waste product formed when the body breaks down purines, substances found in certain foods and in the body's own cells. It is normally dissolved in the blood and excreted through the kidneys into urine. When the body makes too much, or clears too little, levels rise. This is called hyperuricaemia.
According to LML, high levels can lead to conditions such as gout, kidney stones or kidney disease. Uric acid is regulated by transporters in the kidney and gut, which explains why genes, kidney function and some medicines all play a part (Dalbeth et al., 2021).
From uric acid to gout
A Lancet seminar describes gout as a common and treatable disease caused by the deposition of monosodium urate crystals in and around joints, and names raised serum urate as the most important risk factor (Dalbeth et al., 2021). A review of global epidemiology calls it the most common inflammatory arthritis (Dehlin et al., 2020).
Not everyone with high uric acid develops gout, and gout is not only about the flare. The Lancet seminar stresses that, although it presents as attacks that come and go, gout is a chronic disease. Crystals can remain in the joints between flares.
The NHS notes that gout is more common in men, especially with age, and lists factors that raise the chance of getting it, including:
- Being overweight or drinking alcohol.
- Having been through the menopause.
- Taking diuretics (water tablets) or some blood pressure medicines.
- Having high cholesterol, high blood pressure, kidney problems, osteoarthritis or diabetes.
- Having had surgery or an injury.
How the uric acid blood test is used
When gout is suspected
NICE recommends measuring serum urate in people with signs and symptoms of gout. A level of 360 micromol/litre (6 mg/dl) or more supports the clinical picture. NICE adds an important detail: if the result is below that level during a flare and gout is still strongly suspected, the test should be repeated at least 2 weeks after the flare has settled.
Why it can look normal during a flare
That advice exists because urate in the blood can drop during an acute attack. A single normal result in the middle of a painful joint does not settle the question. Testing once things have calmed down gives a more reliable picture.
Following treatment
For people taking long-term urate-lowering treatment, NICE recommends aiming for a serum urate below 360 micromol/litre, and considering a lower target, below 300 micromol/litre, for people with tophi, chronic gouty arthritis or ongoing frequent flares. The American College of Rheumatology guideline also supports a treat-to-target approach (FitzGerald et al., 2020). Here, the uric acid test is how progress is measured.
Urine uric acid
Uric acid can also be measured in urine collected over 24 hours. This tells a professional how much uric acid the kidneys are clearing. It is a more specialised test, sometimes used when kidney stones are being looked into or when a specialist wants to understand why levels are high.
What can shift the number
- Timing around a flare, as explained above.
- Alcohol and very large meals, which the NHS lists among triggers for attacks.
- Dehydration, also listed by the NHS as a trigger.
- Medicines, especially diuretics.
- Kidney function, since the kidneys clear uric acid. A kidney check alongside is often useful.
