Serum Amyloid A in Stable COPD Patients is Associated with the Frequent Exacerbator Phenotype

Background We sought to determine whether circulating inflammatory biomarkers were associated with the frequent exacerbator phenotype in stable COPD patients ie, those with two or more exacerbations in the previous year. Methods Eighty-eight stable, severe, COPD patients (4 females) were assessed for exacerbation frequency, pulmonary function, fraction of expired nitric oxide (FENO); inflammatory variables were measured in venous blood. Logistic regression assessed associations between the frequent exacerbator phenotype and systemic inflammation. Results Compared with infrequent exacerbators, frequent exacerbators (n=10; 11.4%) had greater serum concentration (median (25th-75th quartile)) of serum amyloid A (SAA; 134 (84-178) vs 71 (38-116) ng/mL; P=0.024), surfactant protein D (SP-D; 15.6 (9.0-19.3) vs 8.5 (3.6-14.9) ng/mL; P=0.049) and interleukin-4 (IL-4; 0.12 (0.08-1.44) vs 0.03 (0.01-0.10) pg/mL; P=0.001). SAA, SP-D and IL-4 were not significantly correlated with FEV1%predicted or FVC %predicted. After adjusting for sex, age, BMI, FEV1/FVC and smoking pack-years, only SAA remained independently associated with the frequent exacerbator phenotype (OR 1.49[1.09-2.04]; P=0.012). The odds of being a frequent exacerbator was 18-times greater in the highest SAA quartile (≥124.1 ng/mL) than the lowest SAA quartile (≤44.1 ng/mL) (OR 18.34[1.30-258.81]; P=0.031), and there was a significant positive trend of increasing OR with increasing SAA quartile (P=0.008). For SAA, the area under the receiver operating characteristic curve was 0.721 for identification of frequent exacerbators; an SAA cut-off of 87.0 ng/mL yielded an 80% sensitivity and 61.5% specificity. Conclusion In stable COPD patients, SAA was independently associated with the frequent exacerbator phenotype, suggesting that SAA may be a useful serum biomarker to inform progression or management in COPD.