Routine blood tests and probability of cancer in patients referred with non-specific serious symptoms: a cohort study

Esben Næser, Henrik Møller, Ulrich Fredberg, Jan Frystyk, Peter Vedsted

Danish cancer patients have lower survival rates than patients in many other western countries. Half of the patients present with non-alarm symptoms and thus have a long diagnostic pathway. Consequently, an urgent referral pathway for patients with non-specific serious symptoms was implemented throughout Denmark in 2011-2012. As part of the diagnostic workup, a panel of blood tests are performed for all patients referred by their general practitioner (GP) to the urgent referral pathway. In this study, we analysed the probability of being diagnosed with cancer in GP-referred patients with abnormal blood test results. We performed a cohort study that included all patients aged 18 years or older referred by their GP to Silkeborg Regional Hospital for analysis of a panel of blood tests. All patients were followed for 3 months for a cancer diagnosis in the Danish Cancer Registry. The likelihood ratio and post-test probability of subsequently finding cancer were calculated in relation to abnormal blood test results. Among the 1499 patients included in the study, 12.2% were subsequently diagnosed with cancer. The probability of cancer increased with the number of abnormal blood tests. Patients with specific combinations of two abnormal blood tests had a 23-62% probability of cancer. Only a few single abnormal blood tests were linked with a high post-test probability of cancer, and most abnormalities were not specific to cancer. A number of specific abnormal blood tests and combinations of abnormal blood tests markedly increased the probability of cancer being diagnosed. Still, abnormal blood test results should be interpreted cautiously as most are non-specific to cancer. Thus, results from the blood test panel may strengthen the suspicion of cancer, but blood tests cannot be used as a stand-alone tool to rule out cancer.