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ICD-10-PCS Code Expansion

Source: vignettes/articles/pcs_expansion.Rmd
pcs_expansion.Rmd

ICD-10-PCS (Procedure Coding System) codes are 7-character alphanumeric strings. Because each character position encodes a specific attribute of the procedure, a short prefix can represent many valid codes. peridefs includes expand_pcs() to resolve such patterns to their full set of valid 7-character codes.

The FY2026 reference table

peridefs bundles the CMS FY2026 ICD-10-PCS code table (effective October 1, 2025), which contains 79,115 valid codes. expand_pcs() validates all expansions against this table — it will only return codes that appear on the official CMS list.

How patterns work

expand_pcs() works by prefix matching: it strips all trailing x or X characters from the pattern to form a fixed prefix, then returns every valid code that starts with that prefix.

# "0210xxx" → prefix "0210" → all valid codes starting with 0210
cabg_pcs <- expand_pcs("0210xxx")
length(cabg_pcs)
#> [1] 74
head(cabg_pcs)
#> [1] "0210083" "0210088" "0210089" "021008C" "021008F" "021008W"

Because both lowercase x and uppercase X are stripped from the trailing end, the length of the trailing wildcard string does not matter — only the non-wildcard prefix is used for matching. A pattern with no trailing x/X is treated as an exact code lookup:

# Exact code — returned only if it exists in the FY2026 table
expand_pcs("02100Z9")
#> [1] "02100Z9"

If an exact code does not exist in the reference table, an empty vector is returned:

expand_pcs("0000000")
#> character(0)

Multiple patterns

Pass a character vector to expand several patterns at once. Results are the union of all matches, deduplicated:

# CABG with two different approach prefixes
combined <- expand_pcs(c("0210xxx", "0211xxx"))
length(combined)
#> [1] 148

How bundled PCS specs are built

Bundled specs that include ICD-10-PCS codes — such as spec_chd_v1 (coronary revascularization procedures) — use expand_pcs() internally during the build process in data-raw/build_specs.R. The stored code vectors always reflect the FY2026 reference table.


# ICD-10-PCS codes bundled in spec_chd_v1
spec_chd_v1 <- spec_ascvd$components()[['chd_v1']]
spec_chd_v1$get_codes(code_type = "proc_icd10") |> nrow()
#> [1] 575

When CMS releases a new fiscal year update (typically October 1), the package maintainer runs data-raw/download_pcs_codes.R to refresh the reference table, then rebuilds all affected specs.