EFFECTS OF PAPAIN ON THE AGGLUTINATION OF CANINE RED-CELLS WITH SERUMAUTOANTIBODIES

The papain test, which detects incomplete anti-red blood cell (RBC) au toantibodies in serum, was positive in 12 out of 16 anaemic dogs. Posi tive results were significantly correlated (chi2=11.1, p<0.001) With i ncreased levels of RBC-bound immunoglobulin, but in three of these cas es it was considered that a diagnosis of autoimmune haemolytic anaemia could not be justified. Furthermore, enzyme-linked antiglobulin test measurements of RBC-reactive serum IgG were increased in only three of the dogs with a positive papain test. Papainised canine RBC did not c onsistently take up more serum IgG than untreated cells in an indirect enzyme-linked antiglobulin test. However, the zeta potential of the R BC was reduced after enzyme treatment, and electrophoretic analysis re vealed that glycophorins, which bear a strong negative charge, were cl eaved from the cell membrane. It is concluded that a positive papain t est alone is not reliable in the diagnosis of canine autoimmune haemol ytic anaemia and that the enzyme increases the agglutinability of RBC by reducing the mutually repulsive electrostatic forces between the ce lls, rather than by increasing the amount of autoantibody bound.