The assessment of blood copper status in cattle: A comparison of measurements of caeruloplasmin and elemental copper in serum and plasma

AIM: To evaluate the effect of test, either copper (Cu) concentration or caeruloplasmin (CP) activity, and sample type, either serum or plasma, on the diagnosis of blood Cu status in cattle. METHODS: Paired serum and heparinised plasma samples taken from 125 cattle in 13 herds were tested for Cu concentration and CP activity. The individual results for serum Cu concentration and serum and plasma CP activities were compared with the plasma Cu concentration results, as were their diagnostic values as determined by reference ranges, i.e. 'marginal', 'adequate', 'excess'. RESULTS: The overall mean serum Cu concentration was 2.92 umol/L lower than the mean plasma Cu concentration; however, there was significant variability between individual samples, and the 95% limits of agreement ranged from 0.44 umol/L more to 6.28 umol/L less. The relationship between CP activity and plasma Cu concentration was less variable; the 95% prediction interval for plasma Cu concentration from CP activity was ± 2.8 μmol/L, and was unaffected by whether CP activity was measured in plasma or serum. Using the threshold currently recommended for 'marginal' status of <8.0 μmol/L for serum Cu concentration identified a significantly different population of cattle tlian a threshold of <9.0 μmol/L for plasma samples. Altering the threshold to <7.0 μmol/L for serum Cu concentration produced better agreement. For CP activity, a threshold of 15 IU/L for both serum and plasma identified the same population as a threshold of <9 μmol/L for plasma Cu concentration. CONCLUSIONS: Serum Cu concentration is not a suitable substitute for plasma Cu concentration for the detection of 'marginal' blood Cu status in cattle as the individual variability in the apparent loss of Cu during clotting is too great. In this study, CP activity, in both serum and plasma, was found to be a suitable substitute for the detection of 'marginal' blood Cu status. CLINICAL RELEVANCE: The use of serum Cu concentration rather than plasma Cu concentration in the diagnosis of Curesponsive disease in cattle needs to be re-evaluated as does the way in which individual sample results are used in such tests.