I know with my Bianca they diagnosed her through a bone marrow biopsy. Also the symptoms and the rapid onset gave them a clue. Her cancer was in her bone marrow thus the blood work they continued to do was not giving them any answers = x-rays were clear, but there was something definitely wrong with her. The doctors who did the bone marrow biopsy gave me a slide to keep for comparison later - let's hope I NEVER need it - but there were no normal cells in her marrow - they were full of histocytes - a type of cell. There should have been a variety of cells present that continue on to become various, healthy blood cells but in her sample, there were 90-95% histiocytes and what other cells were present were not that identifiable.
So I know in my particular case, the bone marrow told the tale. Evidently the huge presence of histiocytes, the quick degeneration of blood cells, and her rapid decline brought them to a diagnosis of malignant histiocytosis. Now I am not sure how they determine MH through blood tests on dogs that are yet to have the bone marrow involved, but there seem to be certain marker cells that tell the tale. Her peripheral blood also had strange cells that the specialists said were likely histiocytes but without further testing, they could not say for sure - I would assume the further testing is what you are alluding to and often includes a different mixture of stains that look for specific markers in the blood.
Hope this helps. Molly and the gang Charlottesville, VA
