Truth aspects for each method of measurement
| Modality | Face validity | Construct validity | Content validity | Criterion validity |
|---|---|---|---|---|
| Total number of tophi | Yes | Data not available | Only subcutaneous tophi assessed | Data not available |
| Tape measure | Yes | Data not available | Only subcutaneous tophi assessed | Data not available |
| Vernier calliper | Yes | Of the 47 index tophi identified on physical examination, 42 (89%) were also identified on CT16 There was good correlation between measurements for physical and CT assessments (r=0.91, p<0.0001). There was no difference between physical and CT coefficients of variation for interobserver and intraobserver reproducibility.16 | Only subcutaneous tophi assessed | All patients in the longitudinal study of urate-lowering therapy (ULT) had monosodium urate monohydrate (MSU) crystals documented in synovial fluid or tophus.8 However, tophi included in the analysis were not specifically examined for presence of MSU crystals. |
| Digital photography | Yes | Data not available | Only subcutaneous tophi assessed | Data not available |
| Ultrasonography (US) | Yes | MRI confirmed 37/46 (81%) of the nodules reported by US. US detected 31/41 (90%) of the nodules reported to be tophi on MRI.9 Measurement of tophus size by MRI correlated well with US measurements.9 | Subcutaneous and intra-articular tophi assessed | Presence of MSU crystals was assessed in 12 patients with suspected gouty tophi on ultrasonography. Crystals observed in 10/12 nodules. Remaining two later diagnosed by observation of crystals in synovial fluid samples.9 |
| MRI | Yes | MRI confirmed 37/46 (81%) of the nodules reported by US. US detected 31/41 (90%) of the nodules reported to be tophi on MRI. Measurement of tophus size by MRI correlated well with US measurements.9 | Subcutaneous and intra-articular tophi assessed | In a study of MRI assessment of tophus size, tophi were confirmed by needle aspiration and polarised light microscopy.20 Another study of MRI in gout reported a small mass, not apparent on physical examination or plain radiography, but identified with MRI, was confirmed as a gouty tophus by percutaneous aspiration and polarising light microscopy.18 |
| CT | Yes | For tophi identified by physical and CT assessment, there was good correlation between measurements (r=0.91, p<0.0001)16 | Subcutaneous and intra-articular tophi assessed | Tophi identified by CT were surgically resected and confirmed by histological examination23 |
| Dual energy CT (DECT) | Yes | DECT scans revealed a total of 440 areas of urate deposition in 20 patients with tophaceous gout, compared with only 111 areas on physical examination, p=0.00126 | Subcutaneous and intra-articular tophi assessed | Not systemically analysed. Cases of tophi identified by DECT and confirmed to contain MSU crystals by aspiration and polarised microscopy have been reported.26 |