'I have known patients ask after a blood test if they can be told what their blood group is, and are surprised to discover that this was not tested. Many of our patients appear to believe that a blood test will test for everything - a not unreasonable expectation in these high-tech days. What does this potential misinterpretation really mean? If a doctor performs a blood test to measure the haemoglobin level, and simply informs the patient that his or her blood test was normal, might not the patient assume that everything - including maybe even the HIV status - was normal? If so, the public health implications of blood testing are enormous...'