It involves pushing tips of your index fingers together to see if a diamond-shaped window of light appears between them. If there’s no noticeable gap, it suggests that the nails are ‘clubbed’ - one of the earliest signs of lung cancer, among other serious health issues.
“Any good doctor who sees someone has clubbing will know that they may have some sort of lung disease, heart disease or gastrointestinal disease,” Dr Eric Presser, a thoracic surgeon previously told Health. Clubbing can also be a symptom of other conditions, including pulmonary fibrosis, mesothelioma, Crohn’s disease and colitis.
“If you do notice this change, make sure you talk to your doctor about some of the potential possibilities that you want to rule out,” Dr Presser added.
According to the Mayo Clinic, nail clubbing results when the tops of the fingers or toes enlarge, causing the nail to curve around them as they grow. And while experts can’t confirm exactly why this phenomenon occurs, it’s believed that chronic illnesses trigger a change in the curvature of the nail bed when the blood vessels dilate.
Speaking with Huffington Post, oncology nurse Emma Norton added:
"Most people with lung cancer don’t know that their fingers are clubbing unless they know specifically to look out for it. But the Schamroth window test is a really easy way to check for potential underlying conditions.”
"The test is used by medical professionals as a partial method of confirming conditions, but you can also do the test yourself – and it only takes a few seconds.”