Most studies on serum COMP measurements and osteoarthritis have been performed on patients with knee problems [1-5]. In this COMP letter we present a recent publication by Kelman et a [6]. They measured serum COMP in a community-based cohort of elderly white women. Serum COMP could be used as a risk marker to predict the development of radiographic hip OA in non-diseased subjects. We also present two earlier publications of interest to be discussed in relation to Kelmans study. In one study by Conrozier et al [7] they measured serum COMP in relation to hip joint space narrowing. The authors showed a relation of baseline COMP value and joint space narrowing after one year.

The last study comes from Dragomir et al [8]. They analyzed serum COMP in patients with clinical hip OA but without radiographic alteration. The serum COMP was significantly higher in patients withcsymptoms or clinical signs of hip OA compared to healthy controls.

These three studies indicate that serum COMP measurements could be used as an early predictive marker of hip ostheoarthritis.

Osteoarthritis (OA) of the hip and of the knee is one of the most common chronic diseases of the elderly. It has been shown that nearly 50% of individuals older than 65 years experience OA radiographic changes in either knee or hip. Loss of cartilage integrity is a hallmark of OA. The authors of this study point out the significant need for biomarkers that can easily be obtained from serum to reflect cartilage metabolism and to function as predictors of either the onset of OA at an early stage or progression of radiographic OA.

The aim of the study was to investigate if baseline serum concentrations of COMP were increased in subjects who had developed radiographic OA or showed a progression of
radiographic OA at the follow-up.

The study was based on a community-based cohort (> 65 years, n=5 928) of white women recruited to investigate risk factors for osteoporotic fractures, and not selected primarily for hip OA. The cohort subjects had both a baseline and a follow-up (mean 8.3 years) pelvic radiograph.

By using logistic regression they could show that the risk to develop radiographic OA in non-diseased subjects increased with 30% for each increase of 1 SD of serum COMP at baseline.

Non-diseased individuals with the 25% highest COMP at baseline had a significantly higher risk (adjusted OR 1.67, p < 0.04) to develop radiographic OA than subjects with lower COMP levels.

A similar trend was shown for progression but did not reach significance. The authors suggest that this might be due to using standard pelvic radiographs and a lower progression rate in this community-based population, which has been shown in an earlier publication.

In conclusion, the study shows that in elderly white women serum levels of COMP is a predictive biomarker for the development of radiographic OA.

The aim of this study was to evaluate serum COMP concentration as a predictor of disease progression in hip osteoarthritis by using this improved analysis system as an outcome measurement.

Forty-eight patients (mean age 56.4 years) with symptomatic hip OA were recruited to this one-year study. Only patients with superior femoral head migration were selected to simplify and optimize joint space narrowing measurements.

It was shown that serum concentrations, both at baseline (r = 0.4, p = 0.001) and after one year (r = 0.36, p = 0.02), correlated significantly to joint space width. There was no significant increase in serum COMP level over the year in this population. There was a significant correlation (r = 0.38, p = 0.002) between baseline serum COMP level and the decrease in joint space width after one year.

It was shown that the nine patients with the highest COMP levels had a 61.7% (p = 0.022) higher rate of joint space narrowing than patients with lower COMP levels. Expressed in another way, it was shown that the ten patients with the most marked decrease in joint space narrowing (> 1 mm/year) had significantly (p = 0.01) higher serum COMP at baseline.

The authors conclude that serum COMP seems to be a surrogate marker of OA and may be of interest for the detection of patients at risk of rapidly progressive disease in hip OA.

It is well recognized that radiographic OA and joint symptoms may be discordant. Furthermore, there are studies indicating that joint pain may occur before radiographic
OA.

The aim of this study was to investigate the relationship between serum COMP concentration, as a marker of cartilage turnover, and symptoms or clinical signs of hip
and knee pathology among subjects with no radiographic evidence of OA.

Individuals (n=145, mean age 60.2 years) with no evidence of radiographic hip or knee OA were randomly selected from a population-based prospective study. The serum COMP levels were compared between individuals without joint symptoms and individuals with joint pain expressed as clinical signs, symptoms, or according to ACR criteria.

Serum COMP levels were significantly (p < 0.003) higher in presence of symptoms and remained significant (p < 0.046) after adjustment with symptoms from other joints, age, gender and body mass index. Significant difference after adjustment was also found when clinical signs (p < 0.018) and ACR criteria (p < 0.021) were used as outcome measurements.

In contrast to the above, in individuals with knee pain no significant difference in COMP levels were shown in this study.

The authors suggest that the finding of elevated serum COMP associated with high symptoms and clinical signs in absence of radiographic abnormalities in individuals with hip pain may reflect an underlying pathological process characterized by altered cartilage or bone matrix catabolism or synovitis.