The first digit identifies time periods which can be identified in all years: 1 year ago or less (code 1), 1 to 2 years (code 2), 2 to 3 years (code 3), 3+ years (code 4), Never (code 5), and unknown (code 9). The second digit specifies periods specified for only a subgroup of years.

However, users should be aware of how changes in the exclusive and inclusive nature of response categories impacts the comparability of the composite coding scheme. For example, in 2016 and 2017, all response categories are exclusive of the smaller year and inclusive of the larger year (i.e., "More than 1 year, but not more than 2 years ago"). However, beginning in 2019, all response categories are inclusive of the smaller year and exclusive of the larger year (i.e., "From 1 year to less than 2 yrs"). Therefore, someone who received a blood test exactly two years ago could be coded to "22 - More than 1 year, but not more than 2 years ago" in 2017, but could be coded to "32 - From 2 years to less than 3 yrs" in 2019. This is important if users choose to use only the first digit of the code, since someone who indicates it has been two years since their last blood test could be collapsed into the "2" category in 2017, but the "3" category in 2019. Users concerned about this level of detail may consider retaining all of the detailed codes instead of only looking at the first digit. Please see the codes tab for the availability of each response category.