The capacity of ICD-10-PCS: How much room is there really?

April 23, 2018 / By Rhonda Butler

At the March C&M meeting, we heard the heartening report that people no longer get all in a dither about the number of ICD-10 codes. That is super good news. It means that when it makes sense to add codes, we can focus on what to add, and not on how many codes are going to result. But therein lies the rub: How to decide when it makes sense to add codes and when it doesn’t?

Unfortunately, it isn’t a simple case of the more the merrier. We can’t “just say yes” to every proposal to add new codes to PCS. Why? What’s the big deal? The big deal is multi-axial structure and hierarchy. If ICD-10-PCS codes were a loosely affiliated combination of letters and numbers, like SNOMED CT identifiers, then the capacity of PCS would not be a topic, ever. PCS is a classification, not a clinical terminology. Simply put, it is designed so that similar procedures have similar codes. As partial codes (aka code families) or simple database query statements, they can be more easily lumped together for data analysis. SNOMED was not designed to be aggregated in this way, so the structure of a SNOMED unique identifier is not as constrained.

Multi-axiaI structure and hierarchy help us humans make sense of a collection of information, but they also place limits on the capacity of the system. I am going to use the Med/Surg section of PCS to show how these limits work in practice.

First, multi-axial structure. If you think of the Med/Surg section as a spreadsheet with seven columns, the axis of classification is just a fancy term for what is called a column header in a spreadsheet. In the case of Med/Surg, the column headers are section, body system, root operation, body part, approach device and qualifier. If you have made a spreadsheet with seven columns and you have put a header on each of those columns, you have declared to the world that whatever you put in an individual cell contains one of whatever is the column header. If the column header for the first column is labeled section, then that is all the first column can be used for.

I know—duh—you know this, everybody knows this. But have you stopped to think about the implications for capacity? The position of a letter or number has meaning, and the fact that you can only put one type of information in a column, and that information must be associated with a single letter or number, limits the codes you can create within the defined structure. Let’s compare that with an axis of classification everyone is familiar with—area code. If you had a spreadsheet containing phone numbers, the area code would be one column of the spreadsheet with the column header area code. So, area code is a single axis of classification, but it is a three-digit number. That means its capacity contains 999 options instead of 34 options for an axis of classification in PCS (the numbers 0-9 and the letters of the alphabet except I and O).

In PCS, the structure is not only multi-axial, it is hierarchical. Hierarchy means a bunch of different things, depending on whether you are talking technical or social, but it basically describes a pecking order of some sort, and pecking order is an easy way to think about how hierarchy works in PCS. In the PCS structure, the section is the big boss at the top of the pecking order. It decides the type of information that goes in the other six axes of classification—in Med/Surg, the other six axes are body system, root operation, body part, approach device and qualifier, but in the section called Extracorporeal or Systemic Therapies, the other six axes of classification are physiological systems, operation, body system, duration, qualifier and qualifier.

Within Med/Surg, the second level in the pecking order is body system. Body system determines what gets included in the third and fourth axis—the root operations relevant to that body system, and the body parts relevant to that body system. For example, the root operation Transplantation is only relevant to certain body parts in a few body systems, such as lungs, liver and kidney. Logically enough, the lung body part values are only available in combination with the body system value Respiratory.

Third in the pecking order is root operation. Root operation gets to decide which approaches, devices and qualifiers are relevant, and this varies greatly from root operation to root operation. For example, in the root operation table 0PD, Extraction of Upper Bones, the only approach value available is Open, the only device value available is Z, and the only qualifier available is Z. (0PD^0ZZ is how it looks in PCS shorthand). This is because the root operation Extraction in this context is only used to report non-excisional debridement down to the level of bone, so all other possible approach, device and qualifier values do not apply. By contrast, the root operation table 0PB Excision of Upper Bones, the table contains all possible approaches and qualifiers for both diagnostic and therapeutic procedures (0PB^[0,3,4]Z[X,Z] in PCS shorthand), because this root operation is meant to capture all procedures that use cutting of some sort to take out a portion of the body part. In many other instances, root operation gets to decide not only which device values are relevant, but also which body parts apply to a specific device value. A really simple example is in 0DH, root operation Insertion into the Gastrointestinal system. The device value Feeding Device is included in the table, and is applied to all body part values except the large intestine, because a feeding tube is not inserted in the large intestine.

So far we’ve only dealt in general with how multi-axial structure and hierarchy together define the limits of PCS capacity. Next blog I’ll tackle the capacity itself—where the system has plenty of capacity, where the system is limited but not a problem (“stable,” if you like that word), and where the system is susceptible to running out of capacity. But I am out of capacity for this blog, so stay tuned.

Rhonda Butler is a clinical research manager with 3M Health Information Systems.