Can dehydration cause high levels of B12 in blood?

No. Chronic myelogeneous leukemia, promyelocytic leukemia, polycythemia vera and also the hypereosinophilic syndrome can result in elevated levels of B12 mostly from increased production of haptocorrin. In addition, because the liver stores b12, liver diseases such as acute hepatitis, cirrhosis, hepatocellular carcinoma and metastatic liver disease can also cause a rise in serum b12.