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References

Variants of the Charlson and Elixhauser indices are cited where they are selected (implementation=, weights=). The same references are kept in common/README.md.

Charlson Comorbidity Index

Charlson 1987 (original index; weights="charlson"): Charlson ME, Pompei P, Ales KL, MacKenzie CR. A new method of classifying prognostic comorbidity in longitudinal studies: development and validation. J Chronic Dis. 1987;40(5):373-83. doi: 10.1016/0021-9681(87)90171-8. PMID: 3558716.

Charlson 1994: Charlson M, Szatrowski TP, Peterson J, Gold J. Validation of a combined comorbidity index. J Clin Epidemiol. 1994;47(11):1245-51. doi: 10.1016/0895-4356(94)90129-5. PMID: 7722560.

Deyo 1992 (deyo): Deyo RA, Cherkin DC, Ciol MA. Adapting a clinical comorbidity index for use with ICD-9-CM administrative databases. J Clin Epidemiol. 1992;45(6):613-9. doi: 10.1016/0895-4356(92)90133-8. PMID: 1607900.

Romano 1993 (romano): Romano PS, Roos LL, Jollis JG. Adapting a clinical comorbidity index for use with ICD-9-CM administrative data: differing perspectives. J Clin Epidemiol. 1993;46(10):1075-9. doi: 10.1016/0895-4356(93)90103-8. PMID: 8410092.

D'Hoore 1996 (dhoore): D'Hoore W, Bouckaert A, Tilquin C. Practical considerations on the use of the Charlson comorbidity index with administrative data bases. J Clin Epidemiol. 1996;49(12):1429-33. doi: 10.1016/s0895-4356(96)00271-5. PMID: 8991959.

Schneeweiss 2003 (weights="schneeweiss"): Schneeweiss S, Wang PS, Avorn J, Glynn RJ. Improved comorbidity adjustment for predicting mortality in Medicare populations. Health Serv Res. 2003;38(4):1103-20. doi: 10.1111/1475-6773.00165. PMID: 12968819.

Sundararajan 2004 (australia): Sundararajan V, Henderson T, Perry C, Muggivan A, Quan H, Ghali WA. New ICD-10 version of the Charlson comorbidity index predicted in-hospital mortality. J Clin Epidemiol. 2004;57(12):1288-94. doi: 10.1016/j.jclinepi.2004.03.012. PMID: 15617955.

Quan 2005 (quan, Charlson and Elixhauser): Quan H, Sundararajan V, Halfon P, Fong A, Burnand B, Luthi JC, Saunders LD, Beck CA, Feasby TE, Ghali WA. Coding algorithms for defining comorbidities in ICD-9-CM and ICD-10 administrative data. Med Care. 2005;43(11):1130-9. doi: 10.1097/01.mlr.0000182534.19832.83. PMID: 16224307.

Armitage 2010 (rcs, mapping and weights): Armitage JN, van der Meulen JH; Royal College of Surgeons Co-morbidity Consensus Group. Identifying co-morbidity in surgical patients using administrative data with the Royal College of Surgeons Charlson Score. Br J Surg. 2010;97(5):772-81. doi: 10.1002/bjs.6930. PMID: 20306528.

Quan 2011 (weights="quan"): Quan H, Li B, Couris CM, Fushimi K, Graham P, Hider P, Januel JM, Sundararajan V. Updating and validating the Charlson comorbidity index and score for risk adjustment in hospital discharge abstracts using data from 6 countries. Am J Epidemiol. 2011;173(6):676-82. doi: 10.1093/aje/kwq433. PMID: 21330339.

Thygesen 2011 (thygesen): Thygesen SK, Christiansen CF, Christensen S, Lash TL, Sørensen HT. The predictive value of ICD-10 diagnostic coding used to assess Charlson comorbidity index conditions in the population-based Danish National Registry of Patients. BMC Med Res Methodol. 2011;11:83. doi: 10.1186/1471-2288-11-83. PMID: 21619668.

Ludvigsson 2021 (sweden): Ludvigsson JF, Appelros P, Askling J, Byberg L, Carrero JJ, Ekström AM, Ekström M, Smedby KE, Hagström H, James S, Järvholm B, Michaelsson K, Pedersen NL, Sundelin H, Sundquist K, Sundström J. Adaptation of the Charlson Comorbidity Index for Register-Based Research in Sweden. Clin Epidemiol. 2021;13:21-41. doi: 10.2147/CLEP.S282475. Erratum in: Clin Epidemiol. 2023;15:753-754. doi: 10.2147/CLEP.S425901. PMID: 33469380. Mapping generated from the authors' SAS code.

NHS Digital 2026 (uk_shmi, mapping and weights): NHS Digital. Summary Hospital-level Mortality Indicator (SHMI), specification v1.60, June 2026.

Sokołowski 2026 (sokolowski): Sokołowski PP, Hagmann M, Maros ME, Kamdje Wabo G, Meerjanssen JM, Siegel F. Developing Country-Specific Charlson Comorbidity Index Mappings for Use With German Administrative Data: Methodological Comparative Study. JMIR Med Inform. 2026;14:e93923. doi: 10.2196/93923. PMID: 42593349. Mapping generated from the authors' year-specific mappings.

Elixhauser Comorbidity Index

Elixhauser 1998 (elixhauser, original index): Elixhauser A, Steiner C, Harris DR, Coffey RM. Comorbidity measures for use with administrative data. Med Care. 1998;36(1):8-27. doi: 10.1097/00005650-199801000-00004. PMID: 9431328.

van Walraven 2009 (weights="van_walraven"): van Walraven C, Austin PC, Jennings A, Quan H, Forster AJ. A modification of the Elixhauser comorbidity measures into a point system for hospital death using administrative data. Med Care. 2009;47(6):626-33. doi: 10.1097/MLR.0b013e31819432e5. PMID: 19433995.

Thompson 2015 (weights="thompson_30", "thompson_29"): Thompson NR, Fan Y, Dalton JE, Jehi L, Rosenbaum BP, Vadera S, Griffith SD. A new Elixhauser-based comorbidity summary measure to predict in-hospital mortality. Med Care. 2015;53(4):374-9. doi: 10.1097/MLR.0000000000000326. PMID: 25769057.

Moore 2017 (weights="ahrq" with ICD-9-CM): Moore BJ, White S, Washington R, Coenen N, Elixhauser A. Identifying Increased Risk of Readmission and In-hospital Mortality Using Hospital Administrative Data: The AHRQ Elixhauser Comorbidity Index. Med Care. 2017;55(7):698-705. doi: 10.1097/MLR.0000000000000735. PMID: 28498196.

Sharma 2021 (weights="swiss"): Sharma N, Schwendimann R, Endrich O, Ausserhofer D, Simon M. Comparing Charlson and Elixhauser comorbidity indices with different weightings to predict in-hospital mortality: an analysis of national inpatient data. BMC Health Serv Res. 2021;21(1):13. doi: 10.1186/s12913-020-05999-5. PMID: 33407455.

AHRQ v3.7 (ahrq_icd9): Agency for Healthcare Research and Quality. Elixhauser Comorbidity Software, Version 3.7 (ICD-9-CM).

AHRQ Refined v2026.1 (ahrq_icd10, mapping and weights): Agency for Healthcare Research and Quality. Elixhauser Comorbidity Software Refined for ICD-10-CM, v2026.1.

Combined Comorbidity Score

Gagne 2011 (ICD-9-CM mapping and weights): Gagne JJ, Glynn RJ, Avorn J, Levin R, Schneeweiss S. A combined comorbidity score predicted mortality in elderly patients better than existing scores. J Clin Epidemiol. 2011;64(7):749-59. doi: 10.1016/j.jclinepi.2010.10.004. PMID: 21208778.

Sun 2017 (ICD-10-CM mapping): Sun JW, Rogers JR, Her Q, Welch EC, Panozzo CA, Toh S, Gagne JJ. Adaptation and Validation of the Combined Comorbidity Score for ICD-10-CM. Med Care. 2017;55(12):1046-1051. doi: 10.1097/MLR.0000000000000824. PMID: 29087983.

Hospital Frailty Risk Score

Gilbert 2018: Gilbert T, Neuburger J, Kraindler J, Keeble E, Smith P, Ariti C, Arora S, Street A, Parker S, Roberts HC, Bardsley M, Conroy S. Development and validation of a Hospital Frailty Risk Score focusing on older people in acute care settings using electronic hospital records: an observational study. Lancet. 2018;391(10132):1775-1782. doi: 10.1016/S0140-6736(18)30668-8. PMID: 29706364.