Reader “DenisBH” wondered about our article Should You Be Screened for Melanoma? Check Your Arm, which reported that people with more than 11 moles on their right arm were at higher risk for melanoma.

Since the study’s subjects were British, he asked, would driving rules affect the results? “The study was done in the UK where drivers sit in the right side of the vehicle, opposite to the location of drivers in the US where we sit in the left side of the vehicle,” he writes. “Therefore [for Americans] it is likely to be more helpful to count the number of moles on the left arm…”

Good question! So we asked one of the study’s authors, Simone Ribero, MD, PhD, of the University of Turin, Italy.

Turns out, either the left or the right arm is a good place to count—whichever country you drive in. The right arm was an ever-so-slightly better predictor of the number of moles on your body (over 100 is a warning sign.) In statistical terms, it’s a tiny difference. “The correlation between the two arms is very high,” writes Dr. Ribero. “I don’t think the prediction of the right arm should be influenced by UK driving habits.”

DenisBH had another question, too. By “arm,” do we mean the whole extremity, from the shoulder to the wrist, or just the area from the shoulder to the elbow?

Answer: The whole extremity, from the shoulder to the wrist. Start counting!