Table S2. ICD-9CM and the CCS codes in the primary discharge diagnosis position used to identify causes of 30-day readmissions.

Organ system/diagnosis	CCS codes	ICD-9-CM codes
Respiratory	56, 122–134
Pneumonia and empyema	122
Influenza	123
COPD and asthma	127, 128
Aspiration pneumonitis	129
Pleural disease		510.9, 511.0, 511.89, 511.9, 512.0, 512.89
Respiratory failure		518.8x
Other respiratory tract disease	125,126,133,134
Cardiovascular	96–108, 245
Heart failure	108	402.91, 404.91, 404. 93, 788.51
Conduction disease and arrhythmia	105, 106, 107
AMI	100
CAD	101	414.04
Hypertension	98,99
Pulmonary embolism		415.1x
Syncope	245
Nonspecific chest pain	102
Neurovascular	109–113
Ischemic stroke and transient cerebral ischemia	112	434.11, 434.91
Peripheral vascular	114–121
Hematologic and cancer-related	14, 15, 16, 18, 19, 24, 33, 35, 38, 39, 40, 42, 44, 45, 47, 59–63
Endocrine and metabolic	50, 51, 58
Gastrointestinal	138–141, 143–155
GIB	153
Renal diseases and fluid/electrolyte imbalance	55, 157,158, 160–163
AKI		584.x
Fluid and electrolyte disorders	55
Neuropsychiatric and substance-related	81, 83, 84, 85, 93, 95, 651, 653, 654, 657, 659, 660, 661
MSK and injury related	54, 203–205, 207, 210–212, 226, 229–234, 238–244
Symptoms
Fever of unknown origin	246
Nausea/vomiting	250
Abdominal pain	251
Malaise and fatigue	252
Device, implant, graft complications	237
Others	89, 92, 136, 137, 165–167, 171,175,182,187,189,193, 199, 253, 254, 257, 259

x, the ICD-9CM subcode; ICD-9CM, International Classification of Diseases, Ninth Edition, Clinical Modification; CCS, Clinical Classification Software; COPD, chronic obstructive lung disease; AMI, acute myocardial infarction; CAD, coronary artery disease; UTI, urinary tract infection; AKI, acute kidney injury.