The Luhn Algorithm in C#

PassesLuhnTest.cs

private static bool PassesLuhnTest(string cardNumber)
{
    ReadOnlySpan<char> value = cardNumber;

    var sum = 0;
    var isDouble = false;

    for (var i = value.Length - 1; i >= 0; i--)
    {
        int digit;

        if (value[i] >= 48 && value[i] <= 57)
        {
            digit = value[i] - '0';

            if (isDouble && (digit *= 2) > 9)
            {
                digit -= 9;
            }

            sum += digit;
            isDouble = !isDouble;
        }
    }

    return sum % 10 == 0;
}