We present a low cost, simple and integrated device for medical diagnostics in low-resource settings called the lab-in-a-pen. Finger pricking, and sample collection and processing, are integrated with commercially available paper-based assays in a pen format. This approach ensures safety (i.e. biological sample and sharps containment) and can be used by untrained end users across multiple settings. The pen format also leverages existing low cost, high volume manufacturing and assembly methods. We characterize sample wicking in the lab-in-a-pen using porcine whole blood. The clinical diagnostic utility and usability of the lab-in-a-pen is established by testing of patients for Hepatitis B surface antigen (HBsAg) and Hepatitis B 'e' antigen (HBeAg) by medical staff at the National Hospital for Tropical Diseases in Hanoi, Vietnam.