Is there a variable for sample adults (sample adult file respondents)?

I am using NHIS data from 1994 to 2007 and would like to limit my sample to only sample adults because there are health variables measured among all adults before 1996 but only among sample adults in 1997 and forward. I was not able to find a variable that tells you who are sample adults for the years before 1997. The only variable I was able to find was ASTATFLG (sample adult flag variable) which is available for the year of 1997 and forward but not before 1997. Is this because a sample adult file began from 1997 or is there a variable for sample adults before 1997 which I have missed?

As you have guessed, this is because, prior to 1997, there were no sample adult/child files. The NHIS underwent a major redesign in 1997 and a major part of this redesign was the creation of the sample adult/child concept. Prior to 1997, many of the sample adult/child variables were collected for all household members. More details can be found in the last paragraph of the IPUMS NHIS User Notes on sample design.

I hope this helps.