A bone density of 3.9 is not an indication of seriousness, but rather an indication of severe osteoporosis, and it is recommended that you visit a hospital promptly to clarify the disease. Bone density is an important indicator of the strength of human bones. In medicine, the bone density of the spine and hip joints is usually measured, and is susceptible to differences in measurement due to factors such as gender, age, measuring instruments, and the environment. Generally, a dual-energy x-ray bone densitometer is used to derive the value of bone density. The normal value of bone density is between -1 and 1, while the absolute value of bone density between 1 and 1.5 indicates a decrease in the body’s bone mass, while the absolute value of bone density is greater than 2.5, indicating the presence of osteoporosis symptoms. Therefore, when the bone density is 3.9, it is a severe osteoporosis, and it is easy to produce fracture phenomenon, so we should pay attention to it and go to the hospital in time.