The aim of the present study was to assess the long-term stability of endogenous B-type natriuretic peptide (BNP) and amino terminal proBNP (NT-proBNP) in plasma samples stored at -20 degrees C without addition of protease inhibitors (e.g., aprotinin). Stability of BNP and NT-proBNP was tested in 60 EDTA plasma samples with BNP values between 30 and 420 pg/ml. Initial BNP and NT-proBNP plasma concentrations were determined within four hours after blood collection using the AxSYM BNP and the Elecsys NT-proBNP assays. Subsequently, all samples were stored at -20 degrees C and were thawed for the second BNP and NT-proBNP determination on the two instruments after one day, 30 days, 60 days, 90 days and 120 days, respectively. Mean recovery (i.e., residual immunoreactivity) of BNP and NT-proBNP expressed in percent of the initial value for the given time interval of storage was calculated. Mean recovery of BNP was less than 70% after one day of storage at -20 degrees C and decreased to less than 50% after two to four months of storage (e.g., recovery of endogenous BNP after three months of storage at -20 degrees C ranging from 0% to 71%). In contrast, mean recovery of NT-proBNP was generally greater than 90%, irrespective of the duration of storage at -20 degrees C (e.g., recovery of endogenous NT-proBNP after three months of storage at -20 degrees C ranging from 91% to 112%). In conclusion, the determination of endogenous BNP with the AxSYM assay using frozen plasma samples may not be valid under the conditions tested. In contrast, NT-proBNP as measured by the Elecsys assay may be stored at -20 degrees C for at least four months without a relevant loss of the immunoreactive analyte.