A molecule that changes colour in the blood could provide a simple and accurate way of monitoring drug dosages for patients, say researchers.
The biosensor emits a light that turns gradually from red to blue according to a drug's concentration.
Ensuring patients get the correct doses of medicines can be critically important, especially in cases of cancer, heart disease and epilepsy, and when suppressing the immune system after organ transplants.
But since patients differ in their tolerance to drugs this can be tricky. Too high a dose can lead to unwanted side effects or even poisoning, while one that is too low might not be effective.
Constant monitoring is often needed to get the balance right, which now involves specialist staff and costly testing equipment in laboratories away from the point of care.
The new molecule is customised to bind to a particular drug and contains other elements designed to emit light and alter its colour.
When no drug is present, it generates a red light, but in the drug's presence, molecular changes cause the switch from red to blue.
The signal is measured by placing a drop of blood on a piece of paper and photographing it in a dark box with a conventional digital camera.
Colour-measuring software is then used to produce an average reading from which the drug's concentration in the patient's bloodstream can be calculated.
The sensor molecule can easily be tailored for use with virtually any kind of drug, say its inventors writing in the journal Nature Chemical Biology.
Dr Rudolf Griss, from the Ecole Polytechnique Federale de Lausanne in Switzerland, said: "This system is a cheap, effective solution for customising drug dosage in patients across a whole array of diseases.
"We envision a simple, hand-held detector where the patient can take a pin-prick of blood and can have an immediate reading of free drug concentration in their system - much like diabetics do now for blood glucose."
He and a colleague have launched a start-up company to commercialise the product.
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