For example, maple syrup urine disorder, a genetic condition in which people can’t break down certain parts of proteins, can be diagnosed by sniffing earwax: “It smells of maple syrup,” says Preti. He recently published research revealing that the earwax of people of East Asian descent smells different to that of European, African or American descent, as does their body odour. “We already have an indication that there is some disease-related information in earwax; there’s also information about where you’ve been and what you’ve eaten.” Whether earwax will turn out to be more useful than blood and other body fluids for diagnosing certain conditions remains to be seen, but “we don’t know unless we investigate it,” Preti points out.

Then there’s sweat. For decades it has been used to screen newborns for cystic fibrosis, which alters the balance of sodium and chloride in their sweat. Now, wearable patches are being developed that could alert athletes to changes in the balance of electrolytes that might signal that they’re about to “crash” because of dehydration or physical exertion. One advantage of monitoring sweat is that it could be done passively – you don’t have to stick yourself with a needle, or even mess about with swabs - an electronic sweat-sensor could be worn under clothing that transmits information wirelessly, without you even having to think about it.

But there may also be limitations. “Sweat contains marker molecules, but they are very, very variable, which will probably preclude their direct clinical use,” cautions Jeremy Nicholson, chair in biological chemistry at Imperial College London. Its composition is also influenced by the activity of microbes living on our skin. Blood, on the other hand, tends to provide a more accurate picture of what’s happening inside the body, as it infiltrates every tissue, and the body keeps its basic composition in strict check.

Even within a drop of blood, there’s still a great deal more information that might be gleaned than is currently possible. For instance, Manfred Kayser at Erasmus University Medical Centre in the Netherlands is currently developing new DNA tests that might enable a person’s age, physical appearance, and geographic origin, to be predicted from a blood sample, which could help police identify criminal suspects or badly decomposed bodies.