Image caption Mistakes made while determining someone's blood type may have serious consequences

A self-writing diary in one of J K Rowling's books on Harry Potter has inspired researchers to create a paper that spells out a person's blood type.

A team from Monash University in Australia has developed a paper-based sensor that writes blood type as text.

The sensor may help non-experts to interpret the results rapidly, especially in emergency situations and during humanitarian disasters.

The study appears in the journal Angewandte Chemie.

The device works according to the so-called ABO system, classing blood samples according to A, B, AB or O types, and also spelling out whether the type is Rhesus positive or negative.

According to the system, an A or B letter indicates which antigens are present in red blood cells.

So someone who has, for instance, blood type A has A antigens, and if blood type is AB, then both A and B antigens are present.

People with O blood type have no antigens at all.

Cheaper and faster

At times, people conducting blood tests at home, or even specialists in developing regions, make mistakes while interpreting a blood type test - and these mistakes may have grave consequences, the lead researcher, Professor Wei Shen from Monash University, said to the BBC.

There are places where such strips might be used, such as rapid response scenarios - battlefield casualties, automobile accidents Dr John Brennan , Canada Research Chair in Bioanalytical Chemistry

"We found that more than 80% of the population… could not interpret the result even if the result from a perfectly functioned blood typing assay was presented to them," he said.

"But with a device that can spell out the patient's blood type in written text, people will know their blood type easily."

Having compared the sensor's performance with the mainstream blood typing technologies used in hospitals and pathological laboratories around the world, the team found it has the same accuracy - but it is also cheaper, faster and simpler to use.

These advantages make the sensor ideal for use in developing regions, says Prof Wei Shen.

"Studies show that errors are linked mostly to incorrect registration of the results to the blood sample, or human error," says Prof Wei Shen.

"In developing regions and remote areas, mainstream technologies are not available, and non-mainstream methods are used.

"Misinterpretation of assay results by less-trained health personnel is likely to be a major worry."

And even though the techniques behind the test are the same as conventional methods, "the major novelty is in the ease of reading the strip by spelling out the letters for specific blood types", says Dr John Brennan who holds the Canada Research Chair in Bioanalytical Chemistry.

"I think that there are places where such strips might be used, such as rapid response scenarios - battlefield casualties, automobile accidents, etc - where rapid blood transfusion is required.

"In such cases an unambiguous readout such as that provided by these strips would be important."

Image caption The paper makes blood type letters appear - written in blood

Blood letters

The device consists of a sensor made from a tiny piece of paper, coated with a hydrophobic, water-repellent, layer, but four "windows" are left without it, making them prone to absorb liquid.

Each area is shaped differently; for instance, one has the shape of the letter A, another - the shape of the letter B.

These areas are filled with antibodies that interact with red blood cells, making them clump together, or agglutinate, depending on the blood type.

So when a drop of blood of type A fills the area of the paper containing antibodies corresponding to that type, the red blood cells form clumps and get stuck in the paper fibres, making a letter visible - and the result remains even when the sensor is rinsed.

AB type gives red tint to both A and B-shaped windows.

Since type O has no antigens and thus does not interact with any antibodies, the researchers shaped the third window as the letter X and filled it with antibodies against A and B. They then printed a letter O in the window with red waterproof ink.

Blood types A, B and AB made the X red, eliminating the O type by literally "crossing" it out.

But if the sample is type O, the X becomes white after rinsing it with saline solution, and the red letter O remains.