In the International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) , a letter with two digits followed by a period or decimal point and one number that gives more information on the description of a condition.

Numerical characters in the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) code book after three digits that gives additional specific information about the diagnosis or procedure to be coded. Under Medicare guidelines, some diagnostic and procedure codes require fourth or fifth digits to pass insurance claims processing edits for reimbursement.

Symbol used in the diagnostic code book titled International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) . It is used to indicate that a fourth digit is required for coding to indicate the highest level of specificity.