Update Trigger: COLUMNS_UPDATED Evaluates To True For All String Fields

I have a trigger that writes changes to an audit table. In the case of Updates, I only want to write out the fields that have actually changed. My code for COLUMNS_UPDATED evaluates to true for all varchar fields, even when they haven't changed. All other scenarios appear to be working correctly.



WHILE @field < @maxfield
BEGIN
SELECT @field = min(ORDINAL_POSITION) FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME = @TableName AND ORDINAL_POSITION > @field
SELECT @bit = (@field - 1 )% 8 + 1
SELECT @bit = power(2,@bit - 1)
SELECT @char = ((@field - 1) / 8) + 1

IF ((@TriggerType = 'I') OR (@TriggerType = 'D') OR (@TriggerType = 'U' AND substring (COLUMNS_UPDATED(),@char, 1) & @bit > 0))
BEGIN..........

END

END

Ideas? Thanks!