Slide agglutination testing
This type of test can be tricky. Here’s how to do it right.
Place one drop of blood from a purple-top tube on a slide, visualize it for clumping (agglutination), then add four tiny incremental drops of 0.9% saline with a needle-on syringe to the edges of the blood drop to see if the blood cells disperse or stay clumped. If they stay clumped, the result is positive.
Always look for macroagglutination first (you can see it with your own eyes). If macroagglutination is negative (RBCs either do not clump or disperse quickly with saline), then examine for microagglutination. Microagglutination is the same test but under a microscope, wet and unstained. Use a coverslip, and look for RBCs clinging or clumping together.
For both tests, if the blood cells disperse evenly with the saline, the result is negative. For the microagglutination test it is even more important to use the saline to differentiate agglutination versus rouleaux. Assess the slides immediately; letting the blood sit on the slide for even a few minutes can give the appearance of false-positives as the edges dry out.
If test results are unclear, the ACVIM consensus guidelines recommend washing the blood sample and retesting. Agglutination is caused by cross-linking of IgM immune complexes across RBCs, so it should still be happening after washing.