Blood tests which detect inflammation are not sensitive enough to diagnose serious underlying conditions, generating an 85% false positive rate and a 50% false negative rate when used for this purpose, according to new research.

Many diseases cause inflammation in the body, including infections, autoimmune conditions and cancers. However, research stemming from a collaboration between the University of Bristol and the University of Exeter suggests that inflammatory marker tests are not good at ruling out disease and should not be used for diagnostic purposes by GPs. The results have been published in the British Journal of General Practice.

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University of Bristol senior research fellow Dr Jessica Watson said: “While inflammatory marker tests can contribute to diagnosing serious conditions and are useful for monitoring and measuring response to treatment, their lack of sensitivity means they are not suitable as a rule-out test.”

Using these blood tests to rule out certain conditions can lead to unnecessary or misdirected follow-on consultations, tests and referrals for patients.

Using data from the Clinical Practice Research Datalink, researchers analysed the records of 160,000 patients who had undergone inflammatory marker tests in 2014 and compared these with records of 40,000 patients who had not had the test done.

Of the patients who had ‘raised’ inflammatory marker tests, only 15% were caused by disease: 6.3% were as a result of infections, 5.6% were caused by autoimmune conditions and 3.7% were caused by cancers. The remaining 85% could not be tied to any relevant disease.

The researchers calculated that for every 1,000 inflammatory marker tests performed, 236 false positives would be generated. These would lead to 710 GP appointments, 229 blood tests and 24 referrals in the following six months that did not need to be carried out.

Furthermore, half of the patients whose inflammatory marker tests came back negative did turn out to have a relevant disease.

Watson said: “We recommend that GPs stop using inflammatory markers as a non-specific test to rule out serious underlying disease.”

In April, the same research team published a study in the British Journal of Cancer which found that patients with raised inflammatory marker levels were at greater risk of cancer. The risk was highest in men over 50 and women over 60.

This indicated that GPs should consider cancer as a possible diagnosis in patients with raised inflammatory marker levels, but not rule it out when the levels are within the normal range.

University of Exeter professor of primary care diagnostics Willie Hamilton said: “When we carry out a test in general practice, we have to be clear what we’re looking for. Our study shows it’s unsafe to use one of these tests as a general rule-out for serious disease. They simply don’t do the job.”