3.1. Demographic characteristics
Seventy six and a half percent of the participants were veterans and 23.5% were non-veterans. Of the veterans, 50.3% were drug positive at discharge from the service in 1971 (). Participants were in their mid- to late-40's at the time of VES follow-up interviews in 1996−7. African Americans constituted 16.3% of the follow-up participants; Caucasians, 78.2%, and other, 5.5%. Most of the study participants (82.3%) reported completing high school or completing some college. Most of the participants were married, and 81.5% were employed. Nearly three quarters (71.5%) of the sample were interviewed in person with the remainder interviewed by telephone. Nearly four fifths (78.9%) of the sample reported that they had used no substances in the 90 days preceding the interview. Self-reported substance use in the preceding 90 days ranged from 1.8% for methamphetamine to 17.0% for marijuana. As expected, interview mode (in-person vs. telephone) was associated with hair sampling since hair sampling was not required with long-distance telephone interviews. Otherwise, the participants with hair samples were not significantly different from those without hair samples.
Characteristics of the 25- year follow-up sample (N=839)a
3.2. Agreement between self-reports and confirmation drug hair testing
Preliminary results showed that drug sampling status (D+ and D-) had no significant effect on drug hair testing agreement. Accordingly, results are presented on the pooled data. The detection ratio (the ratio of hair testing prevalence over self-report 90-day use prevalence) varied from 2.15 for cocaine to 0.58 for opiates using self-report as the reference (). The cocaine (p ≤ .0001) and opiate (p ≤ .05) detection ratios were significantly different from one, but the detection ratios for marijuana (p = .68) and methamphetamine (p = .41) were not. Combining the four classes of drugs, the ratio was 1.07, showing that overall hair drug testing was not significantly more likely to detect illicit drug use over the past 90 days. Taking the confirmation hair testing as the reference, specificity was high for self-reports ranging from 94.6 for marijuana, to 100 for cocaine. The overall specificity across the four drugs was 93.4. Sensitivity, however, varied depending on the drug; opiate self-report exhibited the highest sensitivity (77.8), and methamphetamine self-report exhibited the lowest (44.4). In all cases, specificity is much higher than sensitivity, in part because the proportions of positive test results were small. Using self-report as the reference, the specificities were similar to those using the hair testing as the reference, but the sensitivities were lower for marijuana, opiates, and methamphetamines, while it was perfect (100) for cocaine, and slightly higher (73.8) with four drugs combined.
Comparisons between self reports and confirmation drug hair testing (n=613)a
The kappa values for marijuana (κ= .677) and cocaine (κ= .605) suggest self-report and confirmation hair testing were in substantial agreement (usually .6 to .8 range is considered substantial; above .8 is usually considered near perfect). The kappa for opiates (κ= .552) is considered moderate agreement and the kappa for methamphetamines (κ= .370) is considered fair agreement (Landis & Koch, 1977
3.3. Predictors of disagreement between self-report and hair testing
Marijuana and cocaine were the two most commonly-used drugs in this sample. The cocaine disagreement was in the direction of apparent underreporting only (i.e., failing to endorse drug use during the last 90 days, with a positive hair drug test for cocaine). Marijuana disagreements were observed for both apparent under- and overreporting relative to hair results.
Bivariate analysis in shows that participants with apparent underreporting of past 90-day cocaine use were more likely to be African American (χ2 (1, n = 609) =79.20, p < .001) and were less likely to be Caucasian (χ2 (1, n = 609) = 72.31, p < .001), married (χ2 (1, n= 608) = 4.90, p < .05), and employed (χ2 (1, n = 609) = 11.94, p < .001) (, left column). They were more likely to report frequent cocaine use since 1972 (χ2 (1, n = 609) = 7.69, p < .01), to smoke the substance (χ2 (1, n = 608) = 9.94, p < .01), and to have had their last quit attempts involving inpatient care (χ2 (1, n = 609) = 12.72, p < .001). These participants were also more likely to have chemically treated hair (χ2 (1, n = 227) = 4.15, p < .05) and were more likely to provide a short (i.e., < 3cm) hair sample (χ2 (1, n = 609) = 23.21, p < .001). Apparent underreporting of marijuana (i.e., self-report was negative and hair test was positive; , middle column) was significantly related only to frequent marijuana use since 1972 (χ2 (1, n = 513) = 4.83, p < .05). Apparent overestimate of marijuana by self-report (i.e., self-report was positive and hair test was negative; , right column) was associated with being African American (χ2 (1, n = 519) = 17.15, p < .001), with not being Caucasian (χ2 (1, n = 519) = 14.85, p < .001), and being unmarried (χ2 (1, n = 518) = 11.38, p < .001). It is also associated with frequent marijuana use since 1972 (χ2 (1, n = 518) = 27.04, p < .001), having diagnoses of antisocial personality disorder (χ2 (1, n = 519) = 7.10, p < .01) or PTSD (χ2 (1, n = 519) = 8.37, p < .01) since 1972, and providing a short hair sample (< 3cm) (χ2 (1, n = 519) = 17.34, p < .001). All other comparisons were non-significant.
Variables associated with disagreement between self-report and hair testing: cocaine and marijuanaa
Multivariate analyses for disagreement between self-reported cocaine use and positive cocaine hair result (self-report negative and hair drug test positive; left column) yielded an excellent overall fit (Wald Chi-square = 62.7 with p < .0001; c = 0.928). Adjusted odds ratios (AORs) show that this disagreement was significantly associated with younger age, being African American, racial identification as “other,” inpatient care for the participant's most recent quit attempt, and not having ASPD (AOR=0.76, 27.74, 5.16, 5.84, and 0.27, respectively). Although measures associated with disagreement at the bivariate level were correlated, diagnostic analyses did not suggest a high level of multicollinearity among significant predictors.
Adjusted odds ratios (95% confidence intervals) showing major predictors of the disagreement between self-report and hair testing: cocaine and marijuanaa
For the apparent underreporting of marijuana use (i.e., self-report negative and hair drug test positive; middle column), no individual variable was associated with the disagreement between marijuana self-report and hair test (Wald Chi-square = 13.1 with p = .28; c = 0.739). The analysis of apparent overestimate of marijuana use (, right column) yielded an excellent overall fit (Wald Chi-square = 29.9 with p =.0079; c = 0.865). Participants with positive self-report but negative hair testing were also more likely to report frequent marijuana use at some time since 1972 and more likely to provide a short (i.e., < 3cm) hair sample (AOR=27.5 and 3.86, respectively).