Table 2.
The performance of a commercial dot blot compared with immunoprecipitation
| Autoantibody (n) | Dot blot result | Number of times index MSA occurred as false positiveb | Specificity | Sensitivity | Cohen’s κ | ||
|---|---|---|---|---|---|---|---|
| False negative | ‘Unresolved’a | True positive | |||||
| Mi2 (25) | 0 | 6 | 19 | 2 | 0.99 | 0.76 | 0.83 |
| TIF1γ (25) | 18 | 1 | 6 | 5 | 0.99 | 0.24 | 0.31 |
| MDA5 (25) | 6 | 0 | 19 | 7 | 0.98 | 0.76 | 0.73 |
| NXP2 (25) | 2 | 2 | 21 | 5 | 0.99 | 0.84 | 0.81 |
| SAE1 (25) | 1 | 2 | 22 | 4 | 0.98 | 0.88 | 0.85 |
| SAE2 (25) | 10 | 2 | 13 | 5 | 0.98 | 0.52 | 0.58 |
| SAE 1 or SAE2 (25) | 1 | 2 | 22 | 8 | 0.98 | 0.88 | 0.78 |
| Jo-1 (25) | 4 | 0 | 21 | 2 | 0.99 | 0.84 | 0.87 |
| SRP (25) | 3 | 0 | 22 | 10 | 0.97 | 0.88 | 0.75 |
| PL-7 (21) | 0 | 1 | 20 | 2 | 0.99 | 0.95 | 0.93 |
| PL-12 (20) | 1 | 1 | 18 | 2 | 0.99 | 0.90 | 0.81 |
| EJ (10) | 1 | 0 | 9 | 3 | 0.99 | 0.90 | 0.77 |
| OJ (14) | 11 | 2 | 1 | 1 | 1 | 0.07 | 0.12 |
| Zo (9) | 9 | 0 | 0 | 2 | 0.99 | 0 | −0.02 |
| KS (3) | 3 | 0 | 0 | 2 | |||
| Ha (1) | 1 | 0 | 0 | 1 | |||
| Healthy controls (134) | 0 | 11 | 0 | 13c | – | – | – |
| Total (387) | 54 | 26 | 151d | 46e | 0.73 | 0.74f | 0.48 f |
The sensitivity, specificity and Cohen’s κ coefficient for each assay are shown. Tests with κ < 0.8 are highlighted in bold. KS and Ha are included on the dot blot assay but were not analysed further due to very small numbers of immunoprecipitation positive sera available. It is noteworthy that of the three anti-KS samples and one anti-Ha sample available, none tested positive on the assay.
In total, 15 patient samples and 11 healthy control samples were ‘unresolved’ by the assay and a result was unavailable. These were counted as negative for the purposes of sensitivity, specificity and κ calculations.
Some samples contained more than one false positive result. In total, 46 samples contained at least one false positive result.
An additional five samples were positive for anti-Ro52. The 13 listed were positive for myositis-specific autoantibodies. Anti-Ro52 were excluded from specificity calculations because immunoprecipitation is unable to detect this autoantibody, which can be found in healthy individuals.
Samples that contained a true positive result and no false-positive result.
Number of samples analysed containing at least one false positive result.
If ‘unresolved’ results are considered to be false positives, specificity would be reduced to 0.60 and Cohen’s κ to 0.34.