An app has been developed that enables smartphone users to check for anaemia simply by snapping a picture of their fingernails.
The software records levels of haemoglobin, the oxygen-carrying protein in red blood cells, by measuring the paleness of the fingernail bed.
Scientists believe the app will ultimately replace the need for invasive blood tests to monitor blood count.
"All other 'point-of-care' anaemia detection tools require external equipment, and represent trade-offs between invasiveness, cost, and accuracy," Dr Wilbur Lam, whose team of biomedical engineers created the app at Emory University in Atlanta, US, said.
"This is a standalone app whose accuracy is on par with currently available point-of-care tests without the need to draw blood."
Anaemia, caused by low numbers of red blood cells or insufficient haemoglobin, affects two billion people worldwide. Symptoms include pallor, tiredness, and, in severe cases, heart problems.
The new app was developed by matching fingernail photos and blood count haemoglobin levels in 337 people.
Some of the volunteers were healthy while others suffered from a range of conditions leading to anaemia.
Since fingernail beds do not contain the skin pigment melanin, the app can be used by people with both light and dark skins.
The app is designed to take account of light reflections and harmless white patches or "milk spots" on nails known as leukonychia.
The simplicity of the system means it could be especially useful in developing countries, said the researchers, writing in the journal Nature Communications.
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