The Grocery Purchase Quality Index (GPQI) reflects concordance between household grocery purchases and US dietary recommendations. However, it is unclear whether GPQI scores calculated from partial purchasing records reflect individual-level diet quality. This secondary analysis of a 9-month randomised controlled trial examined concordance between the GPQI (range 0-75, scaled to 100) calculated from 3 months of loyalty-card linked partial (≥50 %) household grocery purchasing data and individual-level Healthy Eating Index (HEI) scores at baseline and 3 months calculated from FFQ (n 209). Concordance was assessed with overall and demographic-stratified partially adjusted correlations; covariate-adjusted percentage score differences, cross-classification and weighted κ coefficients assessed concordance across GPQI tertiles (T). Participants were middle aged (55·4 (13·9) years), female (90·3 %), from non-smoking households (96·4 %) and without children (70·7 %). Mean GPQI (54·8 (9·1) %) scores were lower than HEI scores (baseline: 73·2 (9·1) %, 3 months: 72·4 (9·4) %) and moderately correlated (baseline r 0·41 v. 3 months r 0·31, P < 0·001). Correlations were stronger among participants with ≤ bachelor's degree, obesity and children. Scores showed moderate agreement (κ = 0·25); concordance was highest in T3. Participants with high (T3) v. low (T1) GPQI scores had 7·3-10·6 higher odds of having HEI scores >80 % at both time points. Household-level GPQI was moderately correlated with self-reported intake, indicating their promise for evaluating diet quality. Partial purchasing data appear to moderately reflect individual diet quality and may be useful in interventions monitoring changes in diet quality.
Keywords: Diet quality; Grocery purchase data; Grocery purchase quality; Healthy Eating Index.