Is ICD-10-CM really a Diagnosis Coding System?

The answer, perhaps surprising, is no, it is not. ICD-10-CM, like its predecessor, ICD-9-CM, provides codes for categories or classes of diagnoses, but not individual diagnoses.

For example, on page 3 of the 23MB pdf (warning: pdf) that represents ICD-10-CM in its official release format, we find A01.02 Typhoid fever with heart involvement. In the class represented by this code, the file lists two diagnoses:
1. Typhoid endocarditis
2. Typhoid myocarditis

The two diagnoses of typhoid endocarditis and typhoid myocarditis do NOT have their own code in ICD-10-CM. The code A01.02 represents a class of diagnoses, into which at least two diagnoses fall that have no code themselves.

Thus, we see that ICD-10-CM, true to its name, is a classification system. It does not purport to provide codes for individual diagnoses.

A more extreme example is G40.3 Generalized idiopathic epilepsy and epileptic syndromes. Here is a snapshot taken from the ICD-10-CM pdf:

Thus, G40.3 is a class of diagnoses that contains no fewer than 13 individual diagnoses.

Because ICD-10-CM tries to provide a class for every possible diagnosis, present or future, it creates a partition of the diagnosis space. As a result, it requires complex inclusion and exclusion criteria to determine which class or “pigeonhole” each diagnosis falls. These criteria often make it difficult to assign the correct code to a particular patient.

For example, C49 Malignant neoplasm of other connective and soft tissue—and its 15 subclasses—all have the following list of inclusion and exclusion criteria, which span the page break:



Note that, like the rest of ICD-10-CM, none of these inclusion and exclusion criteria are available in a format we can import into a database. Thus, before we can write programs that manipulate these criteria to ensure correct coding, we have to manually type them into our database tables, an error-prone and time-consuming process.

Because of the complexity of assigning a diagnosis to the correct ICD-9-CM category (a situation not ameliorated by ICD-10-CM), the accuracy of data coded with ICD-9-CM suffers. For example, one study found that up to 15-20% of patients classified as having acute stroke did not in fact have a stroke.

Another artifact of the partitional nature of ICD-9-CM and ICD-10-CM is that they both contain wastebasket categories, into which ‘everything else’ under a particular heading goes. For example,

The problem with these types of classes is that their semantics changes over time.

A real-world example of such a change occurred in ICD-9-CM with respect to coding of viral hepatitis. The following chart shows a decline in the incidence of Hepatitis, unspecified beginning about 1981 (open image in a new window to see it more clearly).


This decline was co-incident with the introduction of a code for the class of diagnoses of Hepatitis, Non-A, Non-B. Thus, the true incidence of diseases classified as Hepatitis, unspecified did not change. Rather, the definition of the class itself changed.

These types of wastebasket categories wreak havoc with accurate disease statistics over time. The history of ICD-9-CM is that important diseases such as AIDS and Hepatitis C initially get captured by wastebasket categories, then receive their own codes as they are defined by medical science. The statistics of the incidence and prevalence of these diseases subsequently become quite distorted and difficult to manage.

Yet another problem with ICD-10-CM classes or categories is that they often have criteria that have nothing to with diagnoses or disease, but instead to the timing and nature of the treatment of disease. For example, under the class M48.4 Fatigue fracture of vertebra, we find a requirement to add a 7th character to the code based on (1) whether it is the patient’s first visit to the health care system for such fractures, or a subsequent visit; (2) the rapidity with which the fractures have healed; and (3) whether any complications of such fractures are present:

Wouldn’t it be simpler to switch to a diagnosis coding system where each diagnosis receives its own code?