The depressed subjects were significantly older (mean age=72.4 years, SD=7.8) than the comparison subjects (mean age=67.3, SD=5.4) (t=–2.15, df=71, p=0.04) and more cognitively impaired, according to MMSE scores (mean=26.8, SD=3.3, versus mean=29.1, SD=1.2) (t=4.18, df=50.5, p=0.0001), but they did not differ from the comparison group in percentage of male subjects (26.2% [N=16] versus 33.3% [N=4]) (p=0.73, Fisher’s exact test) or percentage who smoked (15.3% [N=9] versus 0%) (p=0.34, Fisher’s exact test; data missing for two depressed subjects). The mean scores on the Cumulative Illness Rating Scale for heart and vascular disease were low in both groups, indicating minimal cardiovascular disease burden (mean=1.7, SD=1.5, versus mean=1.3, SD=1.3) (t=–1.07, df=71, p=0.29). Platelet factor 4 and β-thromboglobulin levels were significantly higher in the depressed subjects than in the comparison group; the mean platelet factor 4 levels were 34.7 IU/ml (SD=28.4) and 12.7 IU/ml (SD=22.0), respectively (t=–4.30, df=71, p<0.0001), and the mean β-thromboglobulin levels were 119.4 IU/ml (SD=103.1) and 50.1 IU/ml (SD=80.6) (t=–2.95, df=49, p=0.005). The differences remained significant even after we controlled for age (F=15.71, df=1, 70, p=0.0002; Cohen’s r=0.43) and MMSE score (F=15.38, df=1, 70, p=0.0002; Cohen’s r=0.42).
In the 56 depressed subjects classified by 5-HTTLPR polymorphism (t1), platelet factor 4 and β-thromboglobulin levels were highly correlated (r=0.75, p<0.0001, N=39). Depressed subjects with the l/l genotype had significantly higher Hamilton depression scale scores but were otherwise clinically similar to the other depressed subjects. The depressed subjects with the l/l genotype had significantly higher platelet factor 4 and β-thromboglobulin levels. The difference in platelet factor 4 remained significant after we controlled for the difference in Hamilton depression scale score; effect size was indicated by Cohen’s r=0.36.