A total of 570 knee (3 aspects per knee) thermograms were obtained while 190 knees (bilaterally) were scanned by ultrasound among 95 RA patients. The patient baseline characteristics and demographic data were as follows: mean (SD) age was 56.7 (12.9) years; 71 female patients (74.7%); 69 Chinese patients (72.6%); mean (SD) disease duration was 6.6 (5.9) months; mean (SD) DAS28 was 3.80 (1.33); all patients were on conventional disease-modifying anti-rheumatic drug (DMARDs) therapy (i.e. one or more of the following medications: hydroxychloroquine; sulfasalazine; methotrexate; leflunomide) while 66 patients (69.5%) were on oral prednisolone.
Correlation of ultrasound and thermography at the knees
Table 1 summarizes the correlation analysis results between TI parameters and ultrasound outcomes at the bilateral knees. All TI parameters (T-min, T-max and T-avg) correlated significantly with ultrasound GS scores (correlation coefficient ranged from 0.27 to 0.49, P<0.01). Similarly, all TI parameters correlated significantly with ultrasound PD scores (correlation coefficient ranging from 0.21 to 0.43, P<0.05). Specifically, for ultrasound GS scores, the correlation coefficients with the TI parameters were as follows: 0.46, 0.38 and 0.35 for T-max at the right knee lateral, anterior and medial aspects, respectively; 0.35, 0.41 and 0.29 for T-avg at the right knee lateral, anterior and medial aspects, respectively; 0.30, 0.33 and 0.27 for T-min at the at the right knee lateral, anterior and medial aspects, respectively; 0.40, 0.43 and 0.36 for T-max at the left knee lateral, anterior and medial aspects, respectively; 0.47, 0.49 and 0.43 for T-avg at the left knee lateral, anterior and medial aspects, respectively; 0.49, 0.39 and 0.37 for T-min at the at the left knee lateral, anterior and medial aspects, respectively. Specifically, for ultrasound PD scores, the correlation coefficients with the TI parameters were as follows: 0.42, 0.33 and 0.37 for T-max at the right knee lateral, anterior and medial aspects, respectively; 0.30, 0.37 and 0.25 for T-avg at the right knee lateral, anterior and medial aspects, respectively; 0.26, 0.34 and 0.21 for T-min at the at the right knee lateral, anterior and medial aspects, respectively; 0.34, 0.39 and 0.35 for T-max at the left knee lateral, anterior and medial aspects, respectively; 0.40, 0.43 and 0.37 for T-avg at the left knee lateral, anterior and medial aspects, respectively; 0.43, 0.32 and 0.32 for T-min at the at the left knee lateral, anterior and medial aspects, respectively. Overall, some TI parameters showed a weaker correlation while others had a stronger correlation (with correlation coefficients between 0.40 to 0.50 indicating a moderate correlation) with ultrasound-detected joint inflammation.
ROC analysis at the knees
Table 2 and table 3 summarize the results of the ROC analysis for TI parameters (T-min, T-max and T-avg) in identifying PD score > 0 and GS score ≥ 2 at the knees, respectively. For the use of T-min, T-max and T-avg in identifying ultrasound PD score > 0, the area under the ROC curves (AUCs) ranged from 0.63 to 0.82 (see table 2 for the corresponding optimal cut-off, youden’s index, SN, SP, PPV and NPV results). For the use of T-min, T-max and T-avg in identifying ultrasound GS score ≥ 2, the AUCs ranged from 0.65 to 0.82 (see table 3 for the corresponding optimal cut-off, youden’s index, SN, SP, PPV and NPV results). For ultrasound PD joint inflammation (Table 2), TI parameters with AUC > 0.70 to 0.80 are as follows: right knee anterior aspect (T-avg and T-min), lateral aspect (T-max) and medial aspect (T-max); left knee anterior aspect (T-max and T-min), lateral aspect (T-max, T-avg and T-min) and medial aspect (T-max and T-min). For ultrasound PD joint inflammation (Table 2), TI parameters with AUC > 0.80 are as follows: Left anterior aspect (T-avg) and medial aspect (T-avg). For ultrasound GS joint inflammation (Table 3), TI parameters with AUC > 0.70 to 0.80 are as follows: right knee anterior aspect (T-max, T-avg and T-min) and lateral aspect (T-max and T-avg); left knee anterior aspect (T-max and T-min), lateral aspect (T-max and T-min) and medial aspect (T-max and T-min). For ultrasound GS joint inflammation (Table 3), TI parameters with AUC > 0.80 are as follows: Left anterior aspect (T-avg), lateral aspect (T-avg) and medial aspect (T-avg). Overall, there were TI parameters which performed better than the rest, with some attaining an AUC results of > 0.70 to 0.80 or > 0.80.
Intra-observer reliability analysis
For intra-observer reliability analysis, Table 4 summarizes the ICC (and the 95% CI) results for the TI parameters at the knees of patients with RA. The ICC results were high for the TI parameters (T-min, T-max and T-avg) at the knees and ranged from 0.997 to 0.999.