Dear Michelle! Thank you for choosing health care magic. RBC stands for red blood cells. Hb stands for hemoglobin which is located in all red blood cells and plays main role in oxygen carrying from lungs to body organs. There are different conditions which can cause low hemograms like decrease production(Iron deficiency, vitamin deficiency) or increase destruction.(hemolytic diseases) or some times because of bleeding from known or unknown source. Have a detailed discussion with your physician about your condition and the possible therapeutic options.