Wei Shen, a chemical engineer at Australia's Monash University, was pondering the oddities of diagnostics while watching a Harry Potter film at his Melbourne home.

There is a scene in Harry Potter and the Chamber of Secrets where the boy wizard ask a question of Tom Riddle's diary and, magically, the answer comes back in written form.

"Someone asked the paper for something and got a written reply," Wei Shen recalled. "It struck me. Why can't a blood test be like that?"

That eureka moment led to Wei Shen and his Monash colleagues coming up with a blood test the size of a Post-it note and with the texture of a paper towel. Smear a tiny trace of blood on the paper and it comes up with a written answer to the blood-type question of A, B, AB or O and whether the sample is Rhesus positive or negative.

The trick is that in the paper are four spots that contain the antibodies that go with each blood type. The answer, quite literally, shows up in blood-soaked letters because the antibody impregnations are in the form of letters.

The novel sensor, which should go into commercial production soon, addresses the potentially tragic consequences of blood-type sensors that require scientific knowledge to interpret correctly.

"If you build a sensor that doesn't report letters, you still have the problem that even though the sensor works the users will have to have the knowledge of the reaction in order to read it," Wei Shen said.

"If you make this reaction in the form of a letter just like we did, the reporting of the sensor to the user is a lot more straightforward and the key thing is that it's unambiguous."

There are clear advantages to an unambiguous test in poor countries, on the battlefield or in natural disasters. "When you're in a rush, when you're under pressure, you don't want to make a mistake," he said. "This takes away human error."

And with cost pressures in the rich world, the push is on to have patients more involved in diagnostics.

"Simple diagnostics is not going to happen in the hospital anymore," he said. "Simple things can be diagnosed in the home and sent to the doctor by mobile phone or camera or computer."