In this article we describe how patients assign meanings to medical test results and use these meanings to justify their actions. Evidence is presented from lay interpretations of medical tests for monitoring hepatitis C viral infection (HCV) to show how numeracy becomes embodied in the absence of physical symptoms. Illness narratives from 307 individuals infected with HCV were collected from the internet and analysed qualitatively. As part of standard medical care, chronically infected HCV patients are required to have periodic blood tests for laboratory testing. The lab results are presented numerically and compared with established physiological standards. HCV patients’ knowledge and interpretations of test results have important consequences for their health behaviour and their medical decisions. In their stories, the patients described their decisions to begin, delay or stop treatment and developed strategies to alter their diet, exercise and use alternative therapies according to changes in their test result. The perceived meanings of test results are powerful signifiers that are capable of altering the course of HCV patients’ illness, lives and stories. An interpretive model of health numeracy has the advantage of promoting understanding between patients and healthcare providers over a model that views innumeracy as a skill deficit.