: We read with interest the article by Villa and coworkers 1 advocating the use of cystatin C as a measure of glomerular filtration rate (GFR) in critically ill patients. However, we should like to draw attention to several flaws in this study. First, Villa and coworkers compared cystatin C with creatinine as a measure of GFR, using body surface corrected creatinine clearance as, what they call, a 'gold standard'. However, in the Discussion section of that report inulin and iothalamate clearances are mentioned as gold standards, but they were not used by these investigators. The use of body surface area corrected creatinine clearance is questionable in both obese and excessively lean individuals because the correlation between surface area and lean body mass may be lost. Both types of patients are frequently encountered in intensive care. Second, Villa and coworkers employ a cutoff of 80 ml/min to identify renal dysfunction, whereas a value of 50 ml/min is generally accepted 2. This could have a major influence on the presented results. Third, patients with thyroid disorders or on corticosteroid therapy were excluded. Almost all patients with critical illness have low tri-iodothyronine values because of changes in thyroid hormone metabolism ('nonthyroidal illness'), thus making recognition of thyroid disorders problematic. Finally, we showed 3 that, in patients with thyroid dysfunction, cystatin C is not a suitable measure of GFR. In hypothyroidism creatinine levels are elevated but cystatin C levels are low, whereas in hyperthyroidism creatinine levels are low and cystatin C levels elevated. Taken together, we disagree with the authors that cystatin C could be used as a marker of GFR in intensive care patients.