Finally, we applied ultrasound to measure the thickness and area of the rectus femoris muscle in patients with COPD, and as shown in Figures 2E,F, serum GDF15 levels were negatively correlated with RFthick (r = –0.338, p < 0.001) and RFcsa (r = –0.335, p < 0.001). Here, GDF15 is linked to chronic obstructive pulmonary disease.