Comorbid burden was computed by summing the occurrence of 29 different comorbid conditions obtained from the NIS disease severity files. The comorbid conditions examined included: AIDS, alcohol abuse, deficiency anaemias, rheumatoid arthritis/collagen vascular diseases, chronic blood loss anaemia, congestive heart failure, chronic pulmonary disease, coagulopathy, depression, diabetes—uncomplicated, diabetes—with chronic complications, drug abuse, hypertension, hypothyroidism, liver disease, lymphoma, fluid and electrolyte disorders, metastatic cancer, neurological disorders, obesity paralysis, peripheral vascular disorders, psychoses, pulmonary circulatory disorders, renal failure, solid tumour without metastasis, peptic ulcer disease, valvular disease, and weight loss.

NIS, Nationwide Inpatient Sample.