In a TED talk by Abraham Verghese (TED.com), he jokes that if a patient arrives at an American hospital with a leg missing, no one will believe the leg has gone till a scan proves it. It is argued that there is an overreliance in many healthcare settings on medical investigations. Both optometrists and ophthalmologists know that there is a sense of certainty given by a number, for example on a blood test result, and reassurance given by an image, for example by a scan. However we are all also taught early in our clinical training that the history is key. With a good history the list of differential diagnoses can be narrowed down considerably, allowing a thorough examination to point towards one or a few possibilities. Medical investigations, used correctly, are the final step in the diagnostic process. They should only be used to seek confirmation, if necessary, of a suspected diagnosis, to rule out a rare but serious condition or to allow grading of extent or severity of a disease. For example, we use optical coherence tomography (OCT) to demonstrate aspects of age-related macular degeneration which may be treatable and we use the blood test, erythrocyte sedimentation rate (ESR), to rule out giant cell arteritis (GCA): ESR is classically raised in GCA.