The formula for calculating a PLM index is:

The formula for calculating a PLM index is:

• A. TST x 100 / total PLMs
• B. Minutes of sleep / total PLMs x 100
• C. Total PLMs / minutes of sleep x 60
• D. Minutes of sleep / total PLMs x 60

Answer: (C)

The main idea is that the PLM index is a rate: how many periodic limb movements occur per hour of sleep. To get that rate, you take the total number of PLMs during sleep and convert the sleep duration into hours. If you’re using minutes, divide the total PLMs by minutes of sleep to get PLMs per minute, then multiply by 60 to express it per hour. That yields the formula: total PLMs ÷ minutes of sleep × 60. For example, if there are 90 PLMs during 450 minutes of sleep, 90/450 = 0.2 PLMs per minute, and 0.2 × 60 = 12 PLMs per hour. So the PLM index would be 12 per hour. Other options would not reflect the rate per hour. They could produce a percentage of time spent moving, or an inverted ratio that doesn’t represent how many movements occur per hour.

The main idea is that the PLM index is a rate: how many periodic limb movements occur per hour of sleep. To get that rate, you take the total number of PLMs during sleep and convert the sleep duration into hours. If you’re using minutes, divide the total PLMs by minutes of sleep to get PLMs per minute, then multiply by 60 to express it per hour. That yields the formula: total PLMs ÷ minutes of sleep × 60.

For example, if there are 90 PLMs during 450 minutes of sleep, 90/450 = 0.2 PLMs per minute, and 0.2 × 60 = 12 PLMs per hour. So the PLM index would be 12 per hour.

Other options would not reflect the rate per hour. They could produce a percentage of time spent moving, or an inverted ratio that doesn’t represent how many movements occur per hour.