Cardiac peptide stability, aprotinin and room temperature: importance for assessing cardiac function in clinical practice

Brain natriuretic peptide (BNP), atrial natriuretic peptide (ANP) and N-terminal ANP are good research indices of the severity of heart failure. The stability of these peptides at room temperature has become an important factor in assessing their use as indicators of cardiac function in routine clinical practice. Inhibitors such as aprotinin are routinely added in the blood collection process, but may provide no benefit in sample collection and routine clinical practice. We assessed the stability of BNP, ANP and N-terminal ANP in blood samples collected in either the presence or the absence of the protease inhibitor aprotinin. Blood, either with or without aprotinin, was processed immediately (initial; 0 h) and after blood samples had been left for 3 h, 2 days or 3 days at room temperature. These times were chosen to reflect processing in a hospital outpatient clinic (2–3 h), or when posted from general practice (2–3 days). Initial plasma BNP, ANP and N-terminal ANP levels in the absence of aprotinin were 28.2±5.4, 44.2±7.9 and 1997±608 pg/ml respectively, and were not significantly different from initial values in the presence of aprotinin (29.0±5.9, 45.2±8.0 and 2009±579 pg/ml respectively). After 3 h at room temperature, there was a significant fall in ANP in the absence of aprotinin (36.7±7.9 pg/ml; P< 0.005), but not in the presence of aprotinin (41.2±7.6 pg/ml). Both BNP and N-terminal ANP were unchanged in either the absence (BNP, 27.6±5.5 pg/ml; N-terminal ANP, 2099±613 pg/ml) or the presence (BNP, 29.4±5.6 pg/ml; N-terminal ANP, 1988±600 pg/ml) of aprotinin. After 2 days at room temperature, ANP had fallen significantly in both the absence (16.9±3.4 pg/ml) and the presence (24.0±5.0 pg/ml) of aprotinin compared with initial values, and there was a significant difference in ANP levels in the absence and presence of aprotinin (P< 0.001). ANP levels had decreased further after 3 days at room temperature, to 11.9±3.4 pg/ml (no aprotinin) and 20.3±5.0 pg/ml (aprotinin added); these values were significantly different (P = 0.002). In contrast, there was no change in the levels of BNP or N-terminal ANP after 2 or 3 days at room temperature, in either the absence or the presence of aprotinin. These studies indicate that aprotinin adds little benefit to the stability of cardiac peptides at room temperature. Blood samples for BNP and N-terminal ANP measurement used as a test of heart function in hospital clinics and by general practitioners in the community could be taken into blood tubes containing only EDTA as anticoagulant and without the additional step of adding the routinely used inhibitor aprotinin.