The investigators propose to evaluate etCO in patients with HbSS, HbSC, and HbS-beta thalassemia during routine clinic visits, and longitudinally. Our goal is to know whether etCO differs amongst subjects with different sickle cell syndrome genotypes, and whether it is a stable marker of hemolytic rate, as reflected in routine labs obtained for clinical care (including total hemoglobin, reticulocyte count, lactate dehydrogenase, and, when sampled, total and direct bilirubin). We hope to establish whether this inexpensive and non-invasive test faithfully reflects hemolytic parameters in sickle cell syndromes.