To determine the fully specified ICD-9-CM code, interviewers recorded the medical condition reported by the respondent as verbatim text, which was then coded by professional coders. AHRQ provides the following caveat for using ICD9CODE, "Although codes were verified and error rates did not exceed 2.5 percent, analysts should not presume this level of precision in the data; the ability of household respondents to report condition data that can be coded accurately should not be assumed."

Conditions are initially coded "9999" (Unknown) for three reasons: 1) the text indicated that the condition was unknown; 2) the text indicated the condition could not be diagnosed by a doctor; or 3) the specified condition was not codeable and a procedure could not be discerned from the text. When the verbatim text strictly denoted a procedure and not a condition, ICD9CODE was coded to "9996" (NIU).

To preserve confidentiality, the fully specified ICD-9-CM codes have been collapsed to 3-digit code categories and some codes are recoded to broader categories for conditions that occur fewer than 20 times within a year's conditions file and for clinically rare conditions. For pregnancy conditions for a person younger than 16 or older than 44, ICD9CODE was recoded to "9999" (due to the design of the survey most deliveries are coded as pregnancies). In some additional cases, ICD9CODE has been recoded to "9999" for confidentiality reasons.