The WIHS is a multicenter longitudinal observational study of HIV-infected and HIV-uninfected women. Subjects were enrolled at six sites in New York (Bronx and Brooklyn), California (Los Angeles and San Francisco), Washington, D.C., and Chicago. Details of the design are described elsewhere.18
Between October 2004 and March 2007 most women completed a brief neuropsychological testing battery consisting of the Trail Making Test (Parts A and B), a measure of processing speed and cognitive flexibility, and the Symbol Digit Modalities Test (SDMT), a measure of speed of information processing and perceptual motor ability (Analysis 1).19–21
Trail Making and SDMT were examined in relation to concurrent fasting insulin and glucose measures for 503 HIV-uninfected and 1201 HIV-infected subjects with those cognitive measures. Separately, the Comalli version of the Stroop task was completed between October 2006 and September 2008.22
Stroop performance was examined with concurrent fasting insulin and glucose levels available for 347 HIV-negative and 689 HIV-positive participants (Analysis 2).
The parent WIHS selection criteria exclude subjects with clinical dementia at the time of enrollment. All subjects were ambulatory, able to attend an outpatient study visit, and signed Institutional Review Board-approved consent forms. Subjects selected for these analyses met the following additional criteria: no history of diabetes, use of diabetes medication or current fasting glucose over 125
mg/dl, and education level greater than 6 years. Since some subjects had concurrent neuropsychological and fasting insulin/glucose testing done on more than one occasion, we used the first visit in this longitudinal study in which a subject had concurrent fasting insulin levels and cognitive testing. For many subjects, a different visit had to be used for Analysis 2 (Stroop measurements).
Standard detailed medication and medical history were obtained to capture information related to HIV disease stage and metabolic risk factors. Concurrent fasting lipid panels and blood pressures were obtained.
In Part A of the Trail Making Test, subjects were shown a paper displaying individual encircled numbers in a visual array, and they were instructed to rapidly connect the numbers in numerical order. In Part B, the paper displays an array of encircled numbers and letters. Subjects were instructed to draw a line alternating between numbers and letters in alphabetical and numerical order as quickly as possible (e.g., connect “1” to “A” to “2” to “B”). For both Parts, the outcome measure was the number of seconds necessary to complete the test. For the SDMT, subjects were shown a piece of paper displaying a key of nine symbols each paired with a number and below, a rows of numbers on top of empty boxes. Subjects were instructed to fill in the empty boxes with the symbols that correspond to the numbers. The outcome measure was the number of correctly transcribed symbol/number pairs in 90
Trials 1 and 2 of the Stroop served as measures of psychomotor speed and attention. Subjects were instructed to name the color that matches the color within a displayed rectangle (trial 1) and to name the word that matches the displayed word (trial 2). The outcome measure was the number of seconds it takes to complete each trial. Trial 3 of the Stroop served as a measure of executive function, specifically inhibition. Subjects were instructed to name the print color of a series of color names while inhibiting the tendency to read the word (e.g., to say “red” when the word “blue” is shown in red-colored print). The outcome measure was the number of seconds needed to complete the trial.
All neuropsychological test measures were completed in English or Spanish, based on primary language of the subject (95% completed in English). Educational attainment was measured in years of formal education and further estimated IQ by the Wide Range Achievement Test (WRAT).23
The WRAT involves reading recognition, spelling, and arithmetic computation. The WRAT was included as a measure of educational attainment based on evidence that the WRAT and other literacy measures are a more valid index of educational experience than years of school among African Americans.24
HIV status was confirmed by FDA-approved enzyme-linked immunosorbent assay and confirmed with Western blot. Plasma HIV RNA levels and CD4 counts were completed using standard techniques. Insulin resistance was estimated using HOMA, which is defined as (insulin×glucose)/405 with insulin measured in μU/ml and glucose measured in mg/dl.13
Fasting specimens for glucose determination were collected in tubes with glycolytic inhibitors. Serum for insulin determination was obtained at the same time and all specimens were stored (−70°C) until the day of assay. Plasma glucose was measured using the hexokinase method and insulin was measured using the IMMULITE 2000 assay at a central laboratory (Quest Diagnostics, Baltimore, MD).
We used Chi-squared and Mann–Whitney tests to compare variables among participants categorized by HOMA quartiles. For the primary analyses, we utilized raw neuropsychological data without z-transformation, with quartile of HOMA-IR as the primary independent variable of interest. We used quartiles rather than modeling HOMA as a continuous variable because of concerns about nonlinearity and because results would be more easily understandable than using HOMA-IR or a transformation of it as a continuous predictor. Because different visits were used for Analysis 1 and 2, we used different HOMA quartile categories for those analyses in order to keep group sizes approximately equal. Potential confounding variables were analyzed to determine the independent contribution of HOMA to each neuropsychological testing outcome in multivariate linear regression models. We included HOMA in all models and selected additional covariates forward step-wise with p<0.10 required for entry, and then evaluated each remaining unselected covariate as a single addition to the resulting multivariate models. Candidate variables included HIV status, suppression of plasma virus (<1000 copies/ml), hypertension and mean arterial blood pressure, smoking (both current and total pack-years), education, risk for acquiring HIV, current use of cocaine or heroin, language, study site, AIDS diagnosis, hepatitis C RNA positive, on HAART, fasting cholesterol, waist-hip ratio, weight, body mass index, and estimated IQ.