US utility and predictive properties in ACPA and/or IgM-RF-positive arthralgia patients at risk for developing arthritis, but without clinical arthritis, were studied. One-quarter of the patients developed arthritis during follow up and in this group a trend was seen towards more US abnormalities at baseline than in the patients who did not develop arthritis. Furthermore, at the joint level a significant association of US abnormalities and the development of arthritis in that joint was found. Although the positive predictive values for arthritis development of US abnormalities were moderate and only slightly better than for pain at physical examination, the combination of synovitis and power Doppler signal increased predictive properties. Both synovitis and power Doppler signal were better predictors than the presence of joint effusion and the presence of power Doppler signal correlated better with the presence of pain than grey scale US. These results imply that US is able to detect subclinical arthritis in patients that later develop arthritis and are in line with previous studies that showed that US is more sensitive than clinical examination [5
]. The advantage of power Doppler over other modalities has also been shown before. It was more predictive than grey scale US for relapse or radiographic progression in RA patients in clinical remission [13
There are some limitations to this study. First, the interobserver reliability was not optimal. Although overall agreement was good, kappa values were low to moderate. This could partly be explained by the nature of the studied population in which the prevalence of US abnormalities is low and abnormalities, if present, have a low grade. A low prevalence and the difficulty of the differentiation of low-grade abnormalities from normal both decrease interobserver reliability.
A second limitation is that the joints chosen for scanning were selected based on the presence of pain at physical examination or as reported by the patient. Although this resembles a clinical setting, this selection procedure might not be optimal, because this results in different sets of joints and total joint scores cannot be compared.
Finally, at the patient level only a trend was seen towards more development of arthritis in patients that showed US abnormalities. The fact that this trend did not reach statistical significance can partly be explained by low power. The results will need to be confirmed in further studies with larger numbers of patients. On the other hand, US abnormalities were detected in patients that did not develop arthritis. These could be patients that will develop arthritis in the future or abnormalities could have been detected in patients with subclinical inflammation that subsided spontaneously. For clinical decision-making, it is important to discriminate these two patient categories from one another and thus specificity should be increased. This might be achieved by repeating US after a few months. Repeated US could also clarify whether multiple joints show US abnormalities successively in these patients.
Another improvement could be the use of a standardized US joint count with a defined set of joints. Together with regular training sessions, this would probably increase reliability [15
]. A multiple joint scoring system would be more time-consuming, but when only the joints that are mostly afflicted by RA are chosen, this would save time and could nevertheless result in higher sensitivity [5
]. Furthermore, if such a system is able to reliably predict arthritis development, it could be used in the clinic to support treatment decisions. Various simplified US joint count scoring systems have been evaluated in patients with early and established RA that seem feasible to assess joint inflammation in these patient groups [16
]. Such a scoring system will have to be validated for patients presenting with arthralgia.
Alternatively, additional imaging techniques could be used to study subclinical inflammation in seropositive arthralgia patients. Magnetic resonance imaging and positron emission tomography scanning are playing an increasingly important role in the investigation and management of RA [18
]. These techniques could be useful on their own or adjacent to US; their use is currently being investigated in a subgroup of the present cohort.