Do We Need 290 Codes for Diabetes Mellitus?

Despite the emerging genomics revolution that promises to identify the genetic and molecular basis of disease with unprecedented precision, the state-of-the-art science on the nature of diabetes mellitus has identified fewer than 50 subtypes of diabetes mellitus (for example, see the paper Diagnosis and Classification of Diabetes Mellitus).

Nevertheless, ICD-10-CM has approximately 290 codes for diabetes mellitus, not counting diabetes mellitus that arises during the course of pregnancy (also known as gestational diabetes mellitus). We say approximately because again, ICD-10-CM comes as a text document in a pdf. We counted twice and got 289 codes the first time, and 291 codes the second time. These codes span a full 21 pages of the ICD-10-CM document.

So, if we know there aren't 290 types of diabetes mellitus, how does ICD-10-CM derive 290 codes for it?

Combination codes.

A combination code is a code that allows the medical records coder (an entire profession has evolved to review the medical record, apply the rules for assigning billing codes, and create the final set of billing codes submitted to the third-party payer for payment) to assign several diagnoses (or, more properly classes of diagnoses) to a patient in one fell swoop. In addition, it helps to avoid the problem of choosing one diagnosis category as the "primary diagnosis". The coder may assign a combination code as the primary diagnosis, and voila, multiple diagnosis categories are all at once the primary diagnosis, with no messy decisions about which one was the most important or proximate cause of the medical care provided to the patient.

Here is an example of the combination codes created under the heading of diabetes mellitus (click on image to see the whole thing):


Code E11.321 is a combination of two diagnoses, a level of severity, and a physical manifestation of one of the diagnoses: type 2 diabetes mellitus, nonproliferative diabetic retinopathy, mild, and macular edema, respectively. All the possible combinations of types of diabetic retinopathy, severity, and presence/absence of macular edema are present under E11.3 Type 2 diabetes mellitus with ophthalmic complications.

Now, suppose you are a researcher who studies diabetic retinopathy to develop new treatments for this disease, which is the leading cause of blindness in the United States. Suppose further that for a particular study, you were interested in finding all the patients in your data set with nonproliferative diabetic retinopathy.

Instead of searching for all patients with just a single code that represents nonproliferative diabetic retinopathy, you have to locate in the ICD-10-CM pdf all the ICD-10-CM codes that include nonproliferative diabetic retinopathy. Then, you must search on all the codes you locate in this manner.

Nonproliferative diabetic retinopathy is combined with other diagnoses in approximately 50 ICD-10-CM codes. If you miss one, you'll fail to find patients who are potentially eligible for your research study. And since ICD-10-CM is a giant text blob, you cannot rely on the computer to find all 50 codes automatically for you. You have to search the pdf manually.

Combination codes make it hard to use ICD-10-CM encoded data for epidemiology, clinical research, decision support, and any number of other so-called "secondary" uses of medical records data (called secondary uses because the primary use is for the actual care of the patient).

Wouldn't it be better to have one code for one diagnosis? And to assign as many codes as the patient has diagnoses?