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The dehydration for prolonged duration can lead to high RBC count, high hematocrit and high hemoglobin level. But eight-hour fasting cannot lead to such findings. It is advisable to repeat your test after a few days. Until then, keep yourself well hydrated with lots of water. If still, it comes high after a few days, then further workup for polycythemia should be done in form of EPO estimation, chest X-ray, EKG and JAK2 mutation testing as per need.