Comparative yield of Salmonella typhi from blood and bone marrow cultures in patients with fever of unknown origin. Over three years, a comparative study on 100 selected patients with fever of unknown origin was undertaken to determine the yield of Salmonella typhi from their blood and bone marrow cultures. The results indicate that in patients who had an infection with S typhi the organism was isolated from the bone marrow in all of them and from the blood in only 66%. This suggests that bone marrow cultures may be attempted when blood cultures are negative for bacterial growth after three to four days of incubation. 