[Methodological problems in the radioimmunologic measurement of ANP (atrial natriuretic peptide)]

Parameters influencing the measurement of hANP are designated and discussed on the basis of data from the literature and from our own results. Plasma hANP is dependent on the anatomical location of blood withdrawal, posture, state of hydration and salt load and medications. Interfering substances contained in plasma, management of samples and the type of assay used are of utmost importance. Direct RIA of hANP appears to be particularly sensitive to interferences. Preextraction of hANP from EDTA plasma by octadecylsilica cartridges, followed by RIA seems to be the most trustworthy method for measurement of hANP. Nevertheless, with this method too, we obtain different values for identical plasma samples if we use different antisera. Additionally, lipemia significantly lowers the ascertainable amount of endogenous, but not of exogenously added alpha-hANP. Therefore, it is reasonable to conclude that in preextracted samples too, hANP is not a homogeneous species. We observe a time-dependent increase in plasma hANP, reaching a final stable plateau value (about three times the initial values) after about 1.5 h, if EDTA blood samples are left on ice for prolonged time. Hemolysis interferes significantly with hANP determination. Readings of hANP values are reduced in dependence on plasma haemoglobin concentration. According to our experience, EDTA plasma can be stored for several months in either lyophilized form or deep frozen at -20 degrees C without the risk of reduction of hANP immunoreactivity. The data presented in this paper clearly show, that measurement of hANP is susceptible to faults in several details. Results should therefore be viewed critically.