ISBT 128 donation numbers utilize check characters based on the ISO 7064 Mod 37, 2 algorithm. This Annex shows how to calculate the check character for a given number. The calculation is based on the donation number string excluding the leading '=' symbol and the flag characters.

The steps in the process are as follows:

1. For each character in the string determine its check value as required by ISO 7064 (see Table A2.1).
2. For each character determine its weighted check value by multiplying the check value from (1) by the nth power of 2 where n is the position of the character from the right-hand end of the string.
3. Sum the weighted check values from (2).
4. Find the modulus 37 value of the sum from (3).
5. Subtract the value obtained in (4) from 38.
6. Find the modulus 37 value of the result of (5). This is the 37,2 check sum.

The calculated check sum is used to generate both the barcode check characters used in the flag positions of the ISBT 128 barcode and the eye-readable check character. The barcode check characters are determined by adding 60 to the check sum. The eye-readable check character is determined by cross-referencing the checksum to Table A2.1.