Table 1. Overall quality of mapping Read codes to ICD10-CM codes for a subset of 1,313 randomly selected Read codes from GP clinical data.

Number of codes (%)	Issues with mapping quality	Category*
796 (60.6%)	Perfect match: Read code is perfectly matched to ICD10-CM code	No issue
413 (31.4%)	ICD10-CM ontology does not contain a specific code corresponding to a Read code. More general ICD10-CM term has to be used for mapping. (Fig 2B)	Imprecise
36 (2.7%)	Some Read codes describe a group of conditions which can be explained by combinations of several ICD10-CM codes. (Fig 2A)	Multiple
27 (2.0%)	Some Read codes refer to tumor morphology terms. Additional ontology which is specifically for oncology presentation (such as ICD-O-3) should be used to map Read code correctly.	Multiple
16 (1.2%)	There is no ICD10-CM code which would match the Read code precisely. Therefore, the closest ICD10-CM term was selected. (Fig 2C)	Imprecise
15 (1.1%)	Some Read codes refer to general conditions and were mapped to a block of ICD10-CM codes. (For example, Read code “[X]Diseases of inner ear” was mapped to ICD10-CM block “H80-H83 Diseases of inner ear.”)	Multiple
10 (0.7%)	Some Read codes refer to general conditions and were mapped to specific ICD10-CM codes with “unspecified” or “uncomplicated” qualifiers. (For example, Read code “Rubella” was mapped to ICD10-CM code “B069 Rubella without complication.”)	Imprecise

* “Multiple” refers to one-to-many mapping. “Imprecise” means that curators were unable to find precise one-to-one mapping.