
The Chalrson Comorbidity Index is a prognostic Index proposed Mary Charlson et al[1] in 1987 as a means for quantifying the prognosis of patients enrolled in clinical trials.
Premise
Comorbidities are often included in the exclusion criteria while designing trials to avoid the confounding influence on the outcome. However this limits the applicability of the findings to a general population with comorbidity. Inclusion of patients with significant co morbidity can reduce the effectiveness of the treatment making the interpretation of any benefit or harm difficult. A comorbidity index stratifies patients into groups with similar risk of comorbidity induced short term mortality (typically 1 year) making it possible to use it during the sampling and randomization process.
Developement
The Charlson Index was developed using a database of 604 patients admitted in a months period in the New York Hospital. A resident graded the patients illness into grades of mild, moderate, severe and moribund. The patients were followed post discharge for a period of one year. A 93% followup rate was obtained.
Subsequently the a life table analysis was done with patients being censored at the time of death. A steowise backward proportional hazards Cox Regression model was used to calculate the relative hazard of patients with the comorbidity dying. Comorbidities were classifed into 5 categories ranging from absent to moribund and age was categorised in decades.
The adjusted relative hazard rates according to the severity of the condition was then used to produce a weighted index. For example a condition grade with an adjusted hazard rate of greater than equal to 1.2 but less 1.5 would be assigned a score of 1, while when the rate was 2.5 3.5 a score of 3 was assigned. Note that a hazard rate of 1.2 means patient with the condition has a 20% higher risk of dying than one without.
Results
The final index consisted of 19 different conditions assigned into weighted groups as follows. AIDS and metastatic solid tumor had weights of 6. Addition of the scores according the type of morbidity present generates a score. Thus people with a score > 5 have essentially a 100% risk of dying at one year.
A strong influence of age has been seen to exist with comorbidity. Thus it is recommended that 1 point be added to the total score for each decade above the age of 40.[2]
The table below lists the scores for the 19 conditions with the ICD 10 code. This is useful when you have to look up categories like mild liver disease vs moderate to severe liver disease. Just look up the ICD 10 code and you are done. To access ICD10 codes click here
Condition  Score  ICD 10 code 
Myocardial Infarct  1  I21.x, I22.x, I25.2 
Congestive Heart Failure  1  I09.9,I11.0, I13.0, I13.2, I25.5, I42.0, I42.5I42.9, I43.x, I50.x, P29.0 
Peripheral Vascular Disease  1  I70.x, I71.x, I73.1, I73.8, I73.9, I77.1, I79.0, I79.2, K55.1, K55.8, K55.9, Z95.8, Z95.9 
Cerebrovascular Disease  1  G45.x, G46.x, H34.0, I60.x,I69.x 
Dementia  1  F00.x, F03.x, F05.1, G30.x, G31.1 
Chronic Pulmonary Disease  1  I27.8, I27.9, J40.x, J47.x, J60.x, J67.x, J68.4, J70.1, J70.3 
Connective Tissue Disease  1  M05.x, M06.x, M31.5, M32.x, M34.x,M35.1, M35.3, M36.0 
Ulcer Disease  1  K25.x to K28.x 
Mild Liver Disease  1  B18.x, K70.0 to K70.3, K70.9, K71.3 to K71.5, K71.7, K73.x, K74.x, K76.0, K76.2 to K76.4, K76.8, K76.9, Z94.4 
Diabetes  1  E10.0, E10.l, E10.6, E10.8, E10.9, E11.0, E11.1, E11.6, E11.8, E11.9, E12.0, E12.1, El2.6, E12.8, El2.9, E13.0, E13.1, E13.6, E13.8, E13.9, E14.0, E14.1, E14.6, E14.8, E14.9 
Hemiplegia  2  G04.1, G11.4, G80.1, G80.2, G81.x, G82.x, G83.0 to G83.4, G83.9 
Moderate to Severe Renal Disease  2  I12.0, I13.1, N03.2 to N03.7, N05.2 to N05.7, N18.x, N19.x, N25.0, Z49.0 to Z49.2, Z94.0, Z99.2 
Diabetes with End Organ Damage  2  E10.2 to E10.5, E10.7, E11.2, E11.5, E11.7, E12.2 to E12.5, E12.7,E13.2 to E13.5, E13.7, E14.2 to E14.5, E14.7 
Any Tumor  2  C00.x to C26.x, C30.x to C34.x, C37.x to C41.x, C43.x to C45.x, C58.x to C60.x, C76.x to C81.x, C85.x to C88.x, C90.x to C97.x 
Leukemia  2  As Above 
Lymphoma  2  As Above 
Moderate to Severe Liver Disease  3  I85.0, I85.9, I86.4, I98.2, K70.4, K71.1, K72.1, K72.9, K76.5, K76.6, K76.7 
Metastatic Solid Tumor  6  C77.xC80.x 
AIDS  6  B20.xB22.x, B24.x 
What other disadvantages can you think of?
A short review of the Charlson Comorbidity Index has been created at here. Do check out the page and add your comments. The following questions might be interesting for a discussion: