A National Provider Identifier (NPI) is a unique 10-digit identification number issued to health care providers in the United States by the Centers for Medicare and Medicaid Services (CMS).
A 10-digit NPI Number may be validated using the Luhn algorithm by prefixing "80840" to the 10-digit number (80 indicates health applications and 840 indicates the United States).
The Luhn algorithm or Luhn formula, also known as the "modulus 10" or "mod 10" algorithm, is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers, IMEI numbers, NPI numbers in US and Canadian Social Insurance Numbers. It was created by IBM scientist Hans Peter Luhn.
The last digit is the check digit. So the check is performed on the first 9 digits so that the last digit is obtained.
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