We read with interest the article by Sheane et al, published recently in Arthritis Care & Research, in which the authors investigated the importance of ultrasound (US)-detected erosions of the fifth metatarsophalangeal (MTP) joint with respect to the early diagnosis of rheumatoid arthritis (RA) (1). The authors reported that patients with RA, as defined by the American College of Rheumatology (ACR; formerly the American Rheumatism Association) (2), were more likely to have fifth MTP joint erosions (11 of 17 participants) compared with those with undifferentiated arthritis (UA; 6 of 13 participants). We believe that this study and the concept of “target” joints are of great interest. However, we would like to highlight some reasons for being cautious in the interpretation of the results of this study.
There is no consensus on the diagnosis of UA, and this is a difficult problem for the increasing number of very early inflammatory arthritis studies. The criteria employed in this study (early morning joint stiffness >30 minutes and 1 swollen joint) could include noninflammatory (osteoarthritis) patients. There could also be some problems with the design of the study, including the small size of the sample (n = 30), which makes it in essence a pilot study. The study sample also had an unequal number of patients in each disease group (17 with RA, 13 with UA), with no clear standardization of the disease duration in each group.
The authors state that US of the fifth MTP joint “gives a better indication of disease severity and prognosis compared with routinely available laboratory tests,” presumably in the RA cohort. It is not clear what “better” means in this context, since no data are presented to support this statement. The comments relating to prognosis are apparently supported by the fact that US detection of erosions is better than radiographs, which is previously well established (3–7), but no data are presented here to suggest US erosions are associated with a worse outcome in this RA cohort.
With respect to diagnosis, in the current study it does not seem that the presence of a fifth MTP erosion was a strong prognostic marker for the subsequent development of RA. It appears that only 1 of 6 patients in the UA group with baseline fifth MTP erosions subsequently developed RA; this suggests a poor predictive value for this test in terms of diagnosis. Importantly for clinicians, the “added value” of US above routine clinical evaluation was not reported; in the 3 UA patients subsequently diagnosed with RA, what was the rheumatoid factor and anti–cyclic citrullinated peptide (anti-CCP) status? It is also noteworthy that a positive power Doppler (PD) signal was more likely to be found in the RA group, supporting concepts that PD-positivity on US may be a useful diagnostic biomarker for studies of UA (8).