Some factors affecting the measurement of blood inorganic phosphorus in cattle

Sixty cows were studied over 3 days, to determine the effect of site of sampling, handling stress and the tranquilliser xylazine hydrochloride, on blood inorganic phosphorus (P). Jugular venous and coccygeal mixed arterial and venous blood were compared. Saliva was also analysed for P. Buffered and unbuffered trichloracetic acid (TCA) were compared as deproteinising agents. Tail blood P was consistently 12% (range 7-17%) higher than that from the jugular vein (P less than 0.01). This difference decreased when jugular blood was sampled first but increased following administration of the tranquilliser. The drug significantly (P less than 0.01) increased the concentration of P in the saliva on 2 out of 3 days. The possible role of salivary P in affecting jugular P values, is discussed. The buffered TCA analyses gave consistently lower P readings than the unbuffered analyses. The magnitude of this effect was time dependent, being greater when the samples were left for 4 days (P less than 0.05) than when analysed the next day. There was a very strong correlation between plasma and whole blood P. It was concluded that the tissue of choice in phosphorus studies is tail blood (plasma or serum) treated with buffered TCA.