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PPV table, blood tests

The diagnostic value of blood tests in cancer diagnostic in general practice

Positive Predictive Value

The PPV (Positive Predictive Value) table for cancer indicates the probability of whether a patient has cancer when the test result is within a given range.

Our knowledge about the use of blood tests in cancer diagnostics has previously relied on whether the diagnostic value of the blood samples has been at the hospital. However, we need information about the diagnostic value of blood tests when patients present themselves in general practice. At this stage, abnormal blood test results reflect a number of conditions or occur in other contexts than at the hospital. Furthermore, the probability that it actually is a sign of cancer far is lower than in a specialized department.

The tables do not include information about the patient's symptoms, comorbidity, medication status, indication for taking the blood sample or the result of other blood tests that could have influenced the interpretation of the test result. Thus, a positive predictive value for a given blood test result merely be indicative and may be revised up or down depending on other findings.

Likelihood ratio

Likelihood ratio of a blood test result indicates the probability of getting this test result of a patient with cancer in proportion to getting the same test result of a patient without cancer. A likelihood ratio > 1.0 indicates that a blood test result suggests that it is cancer, while a likelihood ratio < 1.0 indicates that a given blood test result suggest that the patient doesn’t have cancer.

Roughly speaking, one can say that the likelihood ratio indicates the degree to which the probability of cancer changes from before to after the test. Likelihood ratio for cancer is therefore used to calculate the probability of cancer (PPV for cancer) at a patient after taking the blood test result into account.