Table 4 –
Estimated association between preoperative anemia (primary exposure), preoperative hemoglobin, and primary and secondary outcomes accounting for effect mediated by total intraoperative RBC transfusions *
| Total effect | Percent mediated | Direct effect | ||||
|---|---|---|---|---|---|---|
| Est. (95% CI) | p-value | Est. (95% CI) | p-value | Est. (95% CI) | p-value | |
| Acute kidney injury | ||||||
| Preoperative anemia | 6.4 (4.2, 8.7) | <.001 | 7.5 (−4.3, 21.1) | 0.220 | 5.9 (3.6, 8.3) | <.001 |
| Preoperative hemoglobin | ||||||
| 14 g/dL | Referent | ‐ | Referent | ‐ | Referent | ‐ |
| 13 g/dL | 1.7 (1.0, 2.3) | <.001 | 4.1 (−4.6, 18.0) | 0.373 | 1.6 (0.9, 2.3) | <.001 |
| 12 g/dL | 3.6 (2.2, 5.1) | <.001 | 5.1 (−5.9, 21.2) | 0.391 | 3.4 (1.8, 5.0) | <.001 |
| 11 g/dL | 5.9 (3.5, 8.4) | <.001 | 6.3 (−8.1, 25.4) | 0.390 | 5.5 (2.9, 8.3) | <.001 |
| 10 g/dL | 8.6 (5.1, 12.3) | <.001 | 8.9 (−10.8, 32.1) | 0.368 | 7.9 (4.1, 11.8) | <.001 |
| 9 g/dL | 11.9 (6.9, 17.5) | <.001 | 11.2 (−14.3, 38.6) | 0.378 | 10.5 (5.3, 16.0) | <.001 |
| Hospital length of stay | ||||||
| Preoperative anemia | 1.07 (1.05, 1.10) | <.001 | 37.9 (22.4, 61.6) | <.001 | 1.04 (1.02, 1.07) | <.001 |
| Preoperative hemoglobin | ||||||
| 14 g/dL | Referent | ‐ | Referent | ‐ | Referent | ‐ |
| 13 g/dL | 1.02 (1.01, 1.03) | <.001 | 24.9 (11.9, 57.9) | <.001 | 1.02 (1.00, 1.03) | 0.006 |
| 12 g/dL | 1.05 (1.03, 1.07) | <.001 | 30.1 (15.2, 63.8) | <.001 | 1.03 (1.01, 1.06) | 0.003 |
| 11 g/dL | 1.08 (1.05, 1.11) | <.001 | 36.0 (19.2, 69.8) | <.001 | 1.05 (1.01, 1.09) | 0.004 |
| 10 g/dL | 1.12 (1.07, 1.17) | <.001 | 42.3 (23.2, 74.8) | <.001 | 1.07 (1.02, 1.12) | 0.006 |
| 9 g/dL | 1.18 (1.12, 1.24) | <.001 | 49.6 (28.7, 79.9) | <.001 | 1.09 (1.02, 1.15) | 0.004 |
Results are from mediation analyses assessing the associations between preoperative anemia/hemoglobin, intraoperative RBC transfusions, and outcomes. The total effect reflects the overall relationship between exposure (e.g., preoperative anemia) and outcome (e.g., postoperative acute kidney injury). This can be partitioned into the portion of the relationship that goes through the mediator (intraoperative RBC transfusion; mediation effect, reported as percentage of total effect) and the portion that does not go through the mediator (direct effect); that is, the total effect is the sum of mediated effect (not shown, rather reported as percentage of total effect) and direct effects subject to rounding error. “Percent Mediated” describes the size of the average causal mediation effects relative to the total effect and, despite being labeled ‘Percent’ the value does not have to be between 0 and 100. Percent mediated values less than 0% indicate the estimated direct effect is greater than the total effect so that mediation is ‘negative’. Percent mediated values greater than 0% indicate the estimated direct effect is less than the total effect (hypothesized in this study). Percent mediated values can also be >100% indicating that the direct and mediated effects are in opposing directions so that the mediated effect is greater than the total effect. Preoperative hemoglobin was parameterized as g/dL below 14, with all values above 14 defined as 0 g/dL below 14. For acute kidney injury, estimates are for the absolute percentage increase in probability of event associated with the given exposure. For hospital length of stay, estimates are for the ratios of geometric mean hospital lengths of stay.