Validating NPI (National Provider Identifier) numbers in SQL

If you work with any kind of healthcare data, then odds are you’ll deal with NPI (National Provider Identifier) numbers. The NPI is a 10-digit number consisting of 9 digits (with the first digit being a 1 or a 2) followed by a check digit. The check digit uses the Luhn algorithm, which is calculated like this:

1. Double the value of alternate digits beginning with the rightmost digit.
2. Add the individual digits of the products resulting from step 1 to the unaffected digits from the original number.
3. Subtract the total obtained in step 2 from the next higher number ending in zero.  This is the check digit.  If the total obtained in step 2 is a number ending in zero, the check digit is zero.

So for example:

Let’s say the 9-digit part of the NPI is 123456789.
NPI without check digit:
1     2     3     4     5     6     7     8     9
Step 1: Double the value of alternate digits, beginning with the rightmost digit.
2            6          10          14          18
Step 2:  Add constant 24, plus the individual digits of products of doubling, plus unaffected digits.
24 + 2 + 2 + 6 + 4 + 1 + 0 + 6 + 1 + 4 + 8 + 1 + 8 = 67
Step 3:  Subtract from next higher number ending in zero.
70 – 67 = 3
Check digit = 3
NPI with check digit = 1234567893 

We needed a way to check our SQL database for valid NPI numbers, so this SQL does the trick:

CREATE FUNCTION  [dbo].[IsValidNPI](@NPI varchar(20)) 
RETURNS int AS

— Returns 1 for valid or missing NPI
— Returns 0 for invalid NPI

— SELECT [dbo].[IsValidNPI](‘1234567893’)
— SELECT [dbo].[IsValidNPI](‘123456789a’)

BEGIN

Declare @Result int, @Len int, @Index int, @Total int, @TmpStr varchar(2), @TmpInt int
   Set @Result = 0
   Set @Total = 0
   Set @NPI = IsNull(@NPI,'')
   Set @Len = Len(@NPI)

   If @Len = 0
      Set @Result = 1
   Else
   Begin
      If @Len <> 10 or IsNumeric(@NPI) = 0 or @NPI like '%.%'
         Set @Result = 0
      Else
      Begin
         Set @Index = @Len

         While @Index > 1
         Begin
            Set @TmpStr = Substring(@NPI, @Index, 1)
            Set @Total = @Total + Cast(@TmpStr as int)

            Set @TmpStr = SubString(@NPI, @Index-1, 1)
            Set @TmpInt = Cast(@TmpStr as int) * 2
            If @TmpInt < 10
               Set @Total = @Total + @TmpInt
            Else
            Begin
               Set @TmpStr = Cast(@TmpInt as varchar(2))
               Set @Total = @Total + Cast(Substring(@TmpStr,2,1) as int)
               Set @Total = @Total + Cast(Substring(@TmpStr,1,1) as int)
            End
            Set @Index = @Index - 2
         End

         If @Len % 2 = 1
            Set @Total = @Total + Cast(Substring(@NPI,1,1) as int)
         If @Len = 10
            Set @Total = @Total + 24

         If @Total % 10 = 0
            Set @Result = 1
         Else
            Set @Result = 0
      End
   End
   Return(@Result)
END