A PPV (positive predictive value) table for a specific cancer disease indicates the probability of whether the patient has the disease when the patient presents the symptom in general practice. The probability increases when the symptom is presented twice, or when the symptom is presented together with another relevant symptom.
Our knowledge about the symptoms of cancer has previously been based on what symptoms cancer patients have presented at the diagnosis in the hospital. However, we need knowledge about the symptoms that are presented in general practice and their predictive value. At this point, the symptom can have a different character or act in a completely different context. Furthermore, the probability that it actually is a sign of cancer is far lower than in a specialized department.
In general practice the tables illustrates that we deal with low prevalence diagnostic. Only few symptoms/signs have a PPV > 5% in general practice. But the tables also have their limitations. A 75 year-old patient with haemoptysis has a bigger risk for lung cancer than a 45 year-old patient with the same symptom which the table does not take into account. Likewise the tables can’t show more than one symptom at the time and does not include the doctor’s gut feeling or the result of diagnostic imaging examinations. Hence, a positive predictive value merely be indicative and may be revised up and down according to the doctor's clinical estimate.
All PPVs are calculated in case-control studies that have used information on symptom presentation in English general practice. Every cancer patient over 40 years was matched with five controls on year of birth, sex and practice association.