Characteristics of participants are summarized in . There was no significant difference between cancers and non-cancers with regard to mean BMI (p=0.20) but some evidence of a shift in the distribution across categories of BMI (p=0.06 in and p=0.03 after combining the OBII and OBIII categories). Among cancers, BMI category did not correlate with high-grade (Gleason score≥7) PCa (p=0.75 and p=0.74 for before and after the OBII/OBIII combination, respectively). The area under the ROC (Receiver Operating Characteristic) curve using continuous BMI was 0.474 (; 95% CI: 0.438-0.510; p=0.92).
Characteristics of Patients in SABOR.
In logistic modeling of PCa risk, there was no significant association between BMI and PCa after adjusting for other risk factors, including age, PSA, DRE, first degree family history of PCa, history of a previous negative biopsy, and race (p=0.35 for continuous BMI; p=0.29 and p=0.18 for categorical BMI before and after the OBII/OBIII combination, respectively).
For men without PCa, serum PSA level was significantly influenced by age, race, BMI, prior negative biopsy, and DRE, and mean PSA decreased significantly with increasing BMI (p<0.001) after adjustment for other covariates (Figures and ). Among cancers, mean PSA increased non-significantly with increasing BMI (p=0.42; Figures and ; p=0.48 after after adjusting for high-grade versus low-grade PCa (p=0.49). The regression of log PSA on BMI category in non-cancers indicated statistically significant reduction with increasing BMI category, with geometric mean PSA of 1.2, 1.1, 1.0, 0.8, 0.7 ng/mL for the under and normal weight, overweight, OBI, OBII and OBIII categories, respectively (F=14.42, df=4, p<0.001). This led to adjustment factors (ratios of under and normal weight relative to each class) equal to: 1.09, 1.20, 1.50, 1.71 for the overweight, OBI, OBII and OBIII categories, respectively. In other words, the unadjusted PSA for men in these weight classes should be multiplied by these factors. After OBII and OBIII were combined, the nearly identical associations between PSA and BMI were observed as described above, and the adjustment factor became 1.5 for the OBII/OBIII category.
Figure 2 Distribution of PSA according to BMI category. Boxes range from the 25th to the 75th quantiles of the distribution with horizontal lines within the boxes denoting the median; whiskers extend from the edge of the box out to 1.5 times the interquartile (more ...)
Distributions of PSA by BMI and racial categories
The AUC for BMI-adjusted PSA values was 0.84, identical to the AUC for unadjusted PSA. The AUC of the PCPT calculator was 0.88, which was slightly better than the AUC of the PCPT calculator with BMI-adjusted PSA of 0.87 (). For those men with a BMI≥35 (OBII/OBIII), the AUCs were 0.88 (95%CI: 0.79 to 0.98), 0.88 (95%CI: 0.78 to 0.98), 0.91(95%CI: 0.82 to 0.99) and 0.90 (95%CI: 0.81 to 0.99) for PSA, BMI-adjusted PSA, the PCPT calculator and the BMI-adjusted PCPT calculator, respectively, which were higher than, but did not differ significantly (p=0.33) from, those men with a BMI<35 (the AUCs were 0.83 [95%CI: 0.81 to 0.86], 0.83 [95%CI: 0.81 to 0.86], 0.87 [95%CI: 0.85 to 0.90] and 0.87 [95%CI: 0.85 to 0.90] for PSA, BMI-adjusted PSA, PCPT calculator and BMI-adjusted PCPT calculator, respectively).
Many prostate biopsy recommendations suggest referral for prostate biopsy if PSA exceeds 2.5 ng/mL or for an abnormal DRE. To post-hoc assess the implication of using BMI-adjusted PSA, the proportion of men with PSA less than or equal to 2.5 ng/mL who would have been considered for prostate biopsy if BMI-adjusted PSA values were used was computed. Of 2816 men with a PSA less than or equal to 2.5 ng/mL who did not receive a biopsy, 126 (4.5%) would have a BMI-adjusted PSA exceeding 2.5 ng/mL. When stratified by race these percentages were 3.9%, 5.1% and 4.1% for Hispanic, white non-Hispanic and black non-Hispanics, respectively, and did not differ significantly (p=0.31).