False-positive results in the detection of methadone in urines of patients treated with psychotropic substances.

later peak (Fig. 1B). The proCNP concentrations in venous plasma from the controls and from the heart failure patients are shown in Fig. 1C. There was no difference between the 2 groups [median (range), 8.3 (7.0–12.0) pmol/L vs 8.0 (4.9–11.0) pmol/L; P 0.47]. The proCNP concentrations in plasma from different cardiovascular regions in the heart failure patients are shown in Fig. 1D. In plasma from the inferior caval vein, the proCNP concentration was marginally higher than in plasma from the coronary sinus [median (range), 7.5 (6.2–11.0) pmol/L vs 8.3 (7.0–12.0) pmol/L; P 0.013]. The present study demonstrates that measurement of precursor peptide is a feasible way to achieve gene product specificity. Extraction by Sep-Pak cartridges, which has been applied in other studies (1, 2, 9), offers the advantage that samples can be concentrated. On the other hand, peptide recovery and, consequently, measurements can be highly variable. To our knowledge, no previous studies have used ultrafiltration for plasma extraction. We established that the recovery is reproducible and sufficient to allow accurate measurement of proCNP. The proCNP concentration in ultrafiltered venous plasma was not increased in heart failure patients compared with controls. The use of plasma proCNP as a marker in stable heart failure patients is thus not supported by our findings. However, patients with destabilized, severe exacerbations were not included in our study. Whether proCNP could be a marker in such patients remains to be investigated. The proCNP concentration was marginally higher in peripheral venous plasma than in plasma sampled from the coronary sinus. Interestingly, cardiac secretion of CNP has been reported recently (10 ). Although our results do not exclude a minor cardiac secretion, they suggest that circulating proCNP is not secreted from the failing heart. Referring to CNP as a “cardiac” natriuretic peptide is thus misleading (11 ).