General practice by numbers: presentation to final outcome.

Abstract

This article explores ways in which numbers may be used in general practice to inform and guide decisions relating to the probabilities of illness, and the harms as well as the benefits of investigations and treatment, both for individual patients and for populations. The numerical approach in the consultation requires an estimate of the probability that the patient with a particular symptom, sign, or test result has a particular disease. This can be expressed as the positive predictive value (PPV) (Figure 1). Once a disease is diagnosed, the additional benefit of treatment compared to no treatment may be expressed as a number needed to treat (NNT) for one person to gain that benefit (Figure 1). An additional term, the number needed to harm, similarly calculated represents the number of individuals treated for one to be harmed by the intervention. The information that provides the basis for these numbers comes from studies in large groups of participants. Increasingly, for diseases in which reduction of the risk of death is not the only criterion of effective care, the quality-adjusted life-year (QALY) or disability-adjusted life-year (DALY) are used as outcome measures. Figure 1 Positive Predictive Value (PPV), Number Needed to Treat (NNT) and Number Needed to Benefit (NNB). For people who are asymptomatic, and for whom systematic population-based screening for a particular condition is proposed, or for patients in highly-selected well-defined groups who reflect participants in randomised controlled trials, NNTs and QALYs can provide the basis for policy decisions on treatment and commissioning. In everyday practice, where the presenting individual is self-selected, has poorly-defined symptoms, and falls between these other two types of populations, different means of measuring and assessing the benefits and harms of pursuing a diagnosis are required. The presentation of patients early in the ‘organisation of their illness’1 is recognised in …