Diagnostic utility of fat infiltration of the SIJ in nr-axSpA versus NSBP patients: mean sensitivity and specificity over 4 readers for cohort A/B (in parentheses for cohort A: nr-axSpA vs NSBP and HC combined).
| Variable | Sensitivity | Specificity | Positive LR | Negative LR |
|---|---|---|---|---|
| FI per se | ||||
| FI in ≥ 1 SIJ quadrant | 0.46 (0.46)/0.44 | 0.72 (0.74)/0.77 | 1.64 (1.79)/1.92 | 0.75 (0.72)/0.73 |
| FI in ≥ 2 SIJ quadrants1 | 0.44 (0.44)/0.42 | 0.73 (0.77)/0.78 | 1.62 (1.91)/1.91 | 0.77 (0.73)/0.74 |
| FI with additional MRI lesions | ||||
| FI w/o BME or erosion | 0.05 (0.05)/0.07 | 0.82 (0.84)/0.92 | 0.28 (0.31)/0.87 | 1.16 (1.13)/1.01 |
| FI w/BME | 0.39 (0.39)/0.27 | 0.91 (0.91)/0.86 | 4.32 (4.35)/1.85 | 0.67(0.67)/0.86 |
| FI w/erosion | 0.34 (0.34)/0.26 | 0.98 (0.99)/0.89 | 17.55 (26.55)/2.43 | 0.68 (0.67)/0.83 |
| Morphological features of FI | ||||
| FI w/distinct border | 0.21 (0.21)/0.21 | 0.97 (0.98)/0.90 | 8.29 (12.54)/2.13 | 0.81 (0.80)/0.88 |
| FI w/distinct border + BME | 0.20 (0.20)/0.12 | 0.99 (0.99)/0.91 | 15.60 (23.60)/1.33 | 0.81 (0.81)/0.97 |
| FI w/distinct border + erosion | 0.18 (0.18)/0.19 | 1.00 (1.00)/0.95 | NC (NC)/3.50 | 0.82 (0.82)/0.86 |
| Homogeneous FI | 0.20 (0.20)/0.26 | 0.97 (0.97)/0.93 | 6.24 (7.87)/3.78 | 0.83 (0.82)/0.80 |
| Homogeneous FI + BME | 0.19 (0.19)/0.16 | 0.99 (0.99)/0.93 | 14.63 (14.75)/2.37 | 0.82 (0.82)/0.90 |
| Homogeneous FI + erosion | 0.15 (0.15)/0.19 | 1.00 (1.00)/0.95 | NC (NC)/3.50 | 0.85 (0.85)/0.86 |
| Subchondral FI | 0.36 (0.36)/0.35 | 0.85 (0.86)/0.83 | 2.36 (2.59)/2.04 | 0.75 (0.74)/0.78 |
| Subchondral FI + BME | 0.31 (0.31)/0.24 | 0.94 (0.94)/0.86 | 4.88 (5.27)/1.77 | 0.73 (0.73)/0.88 |
| Subchondral FI + erosion | 0.26 (0.26)/0.25 | 0.99 (0.99)/0.89 | 20.48 (30.98)/2.36 | 0.75 (0.74)/0.84 |
| FI with ≥ 1 feature2 | 0.36 (0.36)/0.35 | 0.83 (0.84)/0.81 | 2.09 (2.31)/1.87 | 0.77 (0.76)/0.80 |
| FI with ≥ 1 feature + BME | 0.31 (0.31)/0.24 | 0.93 (0.93)/0.86 | 4.43 (4.61)/1.68 | 0.74 (0.74)/0.89 |
| FI with ≥ 1 feature + erosion | 0.26 (0.26)/0.25 | 0.99 (0.99)/0.89 | 20.48 (30.98)/2.36 | 0.75 (0.74)/0.84 |
| FI with ≥ 2 features2 | 0.24 (0.24)/0.30 | 0.97 (0.98)/0.92 | 9.26 (14.01)/3.58 | 0.78 (0.78)/0.77 |
| FI with ≥ 2 features + BME | 0.23 (0.23)/0.19 | 0.99 (0.99)/0.92 | 17.55 (26.55)/2.32 | 0.79 (0.78)/0.88 |
| FI with ≥ 2 features + erosion | 0.19 (0.19)/0.23 | 1.00 (1.00)/0.95 | NC (NC)/4.26 | 0.81 (0.81)/0.82 |
| Anatomical distribution of FI | ||||
| ≥ 1 FI in all 4 sacral quadrants | 0.18 (0.18)/0.19 | 0.90 (0.91)/0.92 | 1.71 (1.88)/2.23 | 0.92 (0.91)/0.89 |
| ≥ 1 FI in all 4 iliac quadrants | 0.04 (0.04)/0.10 | 0.95 (0.96)/0.95 | 0.73 (0.98)/1.82 | 1.01 (1.00)/0.95 |
| ≥ 1 FI in all 4 upper quadrants | 0.06 (0.06)/0.12 | 0.94 (0.95)/0.96 | 1.08 (1.34)/3.19 | 0.99 (0.98)/0.91 |
| ≥ 1 FI in all 4 lower quadrants | 0.06 (0.06)/0.22 | 0.93 (0.95)/0.94 | 0.89 (1.23)/3.59 | 1.01 (0.99)/0.83 |
Differences in positive or negative LR despite identical mean sensitivity and specificity data result from rounding differences.
↵1 FI in ≥ 2 SIJ quadrants on the same slice or in ≥ 1 SIJ quadrant on ≥ 2 adjacent slices.
↵2 Morphological features of FI: FI with distinct border, homogeneous FI, subchondral FI. BME: bone marrow edema; FI: fat infiltration; HC: healthy control; LR: likelihood ratio; MRI: magnetic resonance imaging; NC: not calculable; nr-axSpA: nonradiographic axial spondyloarthritis; NSBP: nonspecific back pain; SIJ: sacroiliac joint; SpA: spondyloarthritis.