Usefulness of quantifying serum KL-6 levels in the follow-up of uveitic patients with sarcoidosis

Background: KL-6 is a human glycoprotein secreted by type II alveolar cells in the lung, and its serum levels increase in pneumonia of various causes. We previously reported that serum KL-6 levels in uveitis patients with sarcoidosis were significantly higher than those of other uveitis patients and healthy controls. Additionally, the combined measurement of serum KL-6 and angiotensin converting enzyme (ACE) was useful for screening uveitic patients to diagnose sarcoidosis. The purpose of the present study was to investigate the clinical usefulness of quantifying serum KL-6 levels for following-up the patients with sarcoidosis. Methods: Sera were obtained from 36 uveitic patients diagnosed with sarcoidosis and the same number of healthy volunteers. To examine the influence of systemic medication, we also collected blood samples from four more sarcoidosis patients, who were systemically treated with corticosteroid or angiotensin converting enzyme (ACE) inhibitor, an anti-hypertensive drug. The serum concentration of KL-6 was measured by a human KL-6 electrochemiluminescence immunoassay (ECLIA). Results: The mean KL-6 concentrations of sarcoidosis patients and healthy controls were 449.3 +/- 66.3 (mean +/- SE) and 192.1 +/- 11.3, respectively. The average levels of serum KL-6 were significantly elevated in sarcoidosis patients compared with healthy control subjects (P < 0.001), and there were significant correlations between serum KL-6 and ACE levels in the patients with sarcoidosis (r=0.70 and P < 0.0001). Moreover, serum KL-6 concentrations were less affected by systemic corticosteroid, and unaffected by ACE inhibitory drugs in contrast to ACE levels. Conclusions: Measurement of serum KL-6 in the uveitic patients may be useful to follow-up the diagnosed sarcoidosis, as well as for diagnosing sarcoidosis, because the serum KL-6 level was well correlated with the ACE level, and less affected by systemic medication than ACE levels.