P140 Comparison of different measures of diffusion capacity in suspected systemic sclerosis associated pulmonary arterial hypertension

Introduction Diffusion capacity of the lung for carbon monoxide (DLCO) incorporates alveolar membrane diffusing capacity (DM) and pulmonary capillary blood volume (VC). DLCO is frequently reduced in systemic sclerosis associated pulmonary arterial hypertension (SSc-PAH) due to a reduction in VC. DLCO may also be reduced in SSc associated pulmonary fibrosis due to lower DM. Carbon monoxide transfer coefficient (KCO = DLCO/VA) and FVC%/DLCO% incorporate lung volumes and so have been suggested as better markers of gas transfer in patients with SSc. Methods 850 SSc patients assessed for suspected PAH between 2001–18 were identified. Co-existent extensive lung disease was defined as moderate-severe fibrosis on CT scan or FVC Results 700 patients had no/limited lung disease (No/Limited-LD) while 150 patients had extensive lung disease (Ext-LD, table 1). Final pulmonary vascular diagnoses were: 492 SSc-PAH, 128 SSc-PH-Lung, 230 no-PH. Correlations with mPAP in No/limited-LD were stronger for DLCO%(-0.47) and KCO% (-0.45) than for FVC%/DLCO% (0.36), p all Conclusion Of the three commonly-used measures of diffusion capacity, DLCO% had the strongest correlation with mPAP and the highest AUC for diagnosing PH in patients with no or limited lung disease. Further analysis to identify the optimal measure for assessing likelihood of PH in patients with extensive lung disease is required.