Table 3.

Relative risk (95% CI) between circulating prolactin concentrations and risk of breast cancer among women who gave two blood samples about 10 years apart, by timing of blood draw in relation to breast cancer diagnosis/reference date in the NHS

Prolactin concentrations (ng/mL)
Prolactin measured <10 years before diagnosis/reference date, proximate
n, cases/controls118/165108/173166/149125/125
All womenb1.000.97 (0.68–1.38)1.64 (1.16–2.32)1.40 (0.96–2.06)0.03
Postmenopausalb1.000.98 (0.63–1.50)1.77 (1.14–2.72)1.75 (1.07–2.88)0.007
Prolactin measured 10–20 years before diagnosis/reference date, distant
n, cases/controls150/176123/160124/140120/136
All womenb1.000.88 (0.62–1.24)0.89 (0.62–1.28)0.83 (0.56–1.22)0.43
Postmenopausalb1.000.80 (0.54–1.19)0.74 (0.48–1.15)0.66 (0.39–1.11)0.11
Average prolactin levels across the two blood collections
n, cases/controls98/124135/201172/166112/121
All womenb1.000.84 (0.59–1.21)1.26 (0.88–1.80)1.03 (0.69–1.54)0.47
Postmenopausalb1.000.69 (0.46–1.04)1.17 (0.77–1.79)1.00 (0.59–1.68)0.42
Percentage change in prolactin levels across the 2 blood collections
n, cases/controls123/152106/154141/153147/153
All womenb,c1.000.94 (0.65–1.37)1.32 (0.90–1.95)1.30 (0.86–1.97)0.13
Postmenopausalb,c1.000.89 (0.54–1.49)1.49 (0.90–2.48)1.53 (0.88–2.66)0.05
Cross-classification of prolactin levels at first and second blood collections
n, cases/controls149/225124/11177/113167/163
All womenb1.001.59 (1.12–2.24)0.94 (0.64–1.38)1.39 (1.01–1.93)
Postmenopausalb1.001.53 (1.03–2.27)0.64 (0.38–1.08)1.41 (0.94–2.12)
  • aP-trend determined using the median values of the quartiles as a continuous term.

  • bUnconditional logistic regression models to calculate RRs and 95% CIs, adjusting for age at first blood draw (continuous), date of first blood draw (continuous), time between first and second blood draw (continuous), fasting status and time of day of both blood draws (fasting and morning draw, other), menopausal status/PMH use at both blood draws (premenopausal/unknown, postmenopausal not using PMH, postmenopausal using PMH), BMI at first blood draw (continuous), change in weight between blood draws (continuous), age at menarche (continuous), history of BBD (yes, no), family history of breast cancer (yes, no), age at menopause at second blood draw (continuous), average childhood body size at ages 5 and 10 (continuous). The analysis of postmenopausal women included 321 cases and 419 controls who were postmenopausal at both blood collections.

  • cModel additionally adjusted for continuous ln-transformed prolactin concentrations at first blood draw.