# ICD Codes

There are four functions in the `medicalcoder` package specifically for working with International Classification of Diseases (ICD) codes.

1. `get_icd_codes()`: returns a look up table of ICD codes as a `data.frame`.
2. `lookup_icd_codes()`: returns details on specific ICD codes.
3. `is_icd()`: returns `TRUE` or `FLASE` for a vector of codes while considering ICD version, type, and billable status.
4. `icd_compact_to_full()`: insert a decimal point into a string to be consistent with ICD-9 diagnostic, ICD-9 procedure, or ICD-10 diagnostic codes. (ICD-10 procedure codes do not have decimal places.) NOTE: this will not validate the return as a valid ICD code, just format the input string appropriately.

# `get_icd_codes()`

A look up table for the ICD codes have been built as internal data sets within the `medicalcoder` package. The sources for these look up tables come from the Centers for Disease Control (CDC) and from the Centers for Medicare & Medicaid Services (CMS). The specific links to the source data sets can be found in the source code for the `medicalcoder` package on GitHub.

    cat(packageDescription('medicalcoder')$URL)

End users can get a `data.frame` with ICD-9 diagnostic, ICD-9 procedure, ICD-10 diagnostic, and ICD-10 procedure codes.

    library(medicalcoder)
    icd_codes <- get_icd_codes()
    str(icd_codes)
    ## 'data.frame':    227534 obs. of  9 variables:
    ##  $ icdv            : int  9 9 9 9 9 9 9 9 9 9 ...
    ##  $ dx              : int  0 0 0 0 0 0 1 0 1 0 ...
    ##  $ full_code       : chr  "00" "00.0" "00.01" "00.02" ...
    ##  $ code            : chr  "00" "000" "0001" "0002" ...
    ##  $ src             : chr  "cms" "cms" "cms" "cms" ...
    ##  $ known_start     : int  2003 2003 2003 2003 2003 2003 1997 2003 1997 2003 ...
    ##  $ known_end       : int  2015 2015 2015 2015 2015 2015 2015 2015 2015 2015 ...
    ##  $ assignable_start: int  NA NA 2003 2003 2003 2003 NA NA 1997 2003 ...
    ##  $ assignable_end  : int  NA NA 2015 2015 2015 2015 NA NA 2015 2015 ...

The columns of this data.frame are:

- `icdv`: integer value 9 or 10 indicating ICD-9 or ICD-10

- `dx`: 1 if the code is a diagnostic code, i.e., from the ICD-9-CM or ICD-10-CM standard. This also covers codes from the World Health Organization (WHO) and the Center for Disease Control and Prevention (CDC) Mortality codes. `dx` will be 0 if the code is a procedure code, i.e., from the ICD-9-PCS or ICD-10-PCS standard.

- `full_code`: the full ICD code with the decimal point if applicable.

- `code`: compact code, any applicable decimal point has been omitted.

- `src`: a character vector denoting the source of the information.

- `known_start`: The first year that the `medicalcoder` package as data for this code.

  - For codes based on the ICD-9-CM, ICD-9-PCS, ICD-10-CM, ICD-10-PCS standards the year is the fiscal year for the United States Federal Government; October 1 - September 30. For example, fiscal year 2018 started October 1, 2017 and ended on September 30, 2018.

  - For codes from the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) Mortality coding, the year is calendar year.

- `known_end`: the last year the code was part of the standard, or that the `medicalcoder` package has data for.

  - ICD-9 last year of active use was FY 2015.
  - ICD-10 is active. The current version of `medicalcoder` has details on ICD-10 codes through FY 2026.

- `assignable_start`: The first year (fiscal or calendar based on src) a code was assignable. `NA` indicates the code was never assignable. Assignable status can vary from source to source. For example the code “C84.6” is an assignable code using the WHO ICD-10 codes because there is no code with more granularity. The same code is not assignable under ICD-10-CM because codes C84.60, C84.61, C84.62, C84.63, C84.64, C84.65, C84.66, C84.67, C84.68, C84.69, C84.6A exist. Codes that are not assignable are called header codes. Ideally codes are reported with the greatest level of granularity, but that is not always the case.

- `assignable_end`: The last year the code was assignable.

To get the descriptions of the ICD codes call `get_icd_codes()` with `with.descriptions = TRUE`.

    str(get_icd_codes(with.descriptions = TRUE))
    ## 'data.frame':    241988 obs. of  12 variables:
    ##  $ icdv            : int  9 9 9 9 9 9 9 9 9 9 ...
    ##  $ dx              : int  0 0 0 0 0 0 1 0 1 1 ...
    ##  $ full_code       : chr  "00" "00.0" "00.01" "00.02" ...
    ##  $ code            : chr  "00" "000" "0001" "0002" ...
    ##  $ src             : chr  "cms" "cms" "cms" "cms" ...
    ##  $ known_start     : int  2003 2003 2003 2003 2003 2003 1997 2003 1997 1997 ...
    ##  $ known_end       : int  2015 2015 2015 2015 2015 2015 2015 2015 2015 2015 ...
    ##  $ assignable_start: int  NA NA 2003 2003 2003 2003 NA NA 1997 1997 ...
    ##  $ assignable_end  : int  NA NA 2015 2015 2015 2015 NA NA 2015 2015 ...
    ##  $ desc            : chr  "Procedures and interventions, Not Elsewhere Classified" "Therapeutic ultrasound" "Therapeutic ultrasound of vessels of head and neck" "Therapeutic ultrasound of heart" ...
    ##  $ desc_start      : int  2003 2003 2003 2003 2003 2003 1997 2003 1997 2010 ...
    ##  $ desc_end        : int  2015 2015 2015 2015 2015 2015 2015 2015 2009 2015 ...

The return has the additional columns:

- `desc`: the description of the code
- `desc_start`: the first fiscal_year that the description was documented
- `desc_end`: the last fiscal_year that the description was documented

Over time the descriptions for some ICD codes were modified within sources. There are also many differences between sources. The table below has several examples.

    delta_in_desc <-
      subset(get_icd_codes(with.descriptions = TRUE),
             subset = full_code %in% c("Z88.7", "010.93", "V76.49"),
             select = c("full_code", "src", "desc", "desc_start", "desc_end"))

full_code | src | desc | desc_start | desc_end
--- | --- | --- | --- | ---
010.93 | cms | Primary tuberculous infection, unspecified tubercle bacilli found (in sputum) by microscopy | 1997 | 2009
010.93 | cms | Primary tuberculous infection, unspecified, tubercle bacilli found (in sputum) by microscopy | 2010 | 2015
V76.49 | cms | Other | 1997 | 1999
V76.49 | cms | Other sites | 2001 | 2009
V76.49 | cms | Special screening for malignant neoplasms of other sites | 2010 | 2015
Z88.7 | cms | Allergy status to serum and vaccine status | 2014 | 2020
Z88.7 | cms | Allergy status to serum and vaccine | 2021 | 2026
Z88.7 | who | Personal history of allergy to serum and vaccine | 2008 | 2019

- Z88.7 has differences in the description over time within `cms` source and between `cms` and `who`.

- The only difference in the description for 010.93 is a comma.

- ICD-9-CM V79.49 had the description of ‘other’ which would require exploration of the header codes to understand. Even when the most verbose description may still require consideration of the header codes to fully understand.

Lastly, the `get_icd_codes()` function includes the argument `with.hierarchy` which will provide additional details for the codes.

    str(get_icd_codes(with.hierarchy = TRUE))
    ## 'data.frame':    227534 obs. of  16 variables:
    ##  $ icdv                : int  10 10 10 10 10 10 10 10 10 10 ...
    ##  $ dx                  : int  0 0 0 0 0 0 0 0 0 0 ...
    ##  $ full_code           : chr  "001" "0016070" "0016071" "0016072" ...
    ##  $ code                : chr  "001" "0016070" "0016071" "0016072" ...
    ##  $ src                 : chr  "cms" "cms" "cms" "cms" ...
    ##  $ known_start         : int  2014 2014 2014 2014 2014 2014 2014 2014 2014 2014 ...
    ##  $ known_end           : int  2026 2026 2026 2026 2026 2026 2026 2026 2026 2026 ...
    ##  $ assignable_start    : int  NA 2014 2014 2014 2014 2014 2014 2014 2014 2014 ...
    ##  $ assignable_end      : int  NA 2026 2026 2026 2026 2026 2026 2026 2026 2026 ...
    ##  $ chapter             : chr  "0  Medical and Surgical" "0  Medical and Surgical" "0  Medical and Surgical" "0  Medical and Surgical" ...
    ##  $ subchapter          : chr  "00 Central Nervous System and Cranial Nerves" "00 Central Nervous System and Cranial Nerves" "00 Central Nervous System and Cranial Nerves" "00 Central Nervous System and Cranial Nerves" ...
    ##  $ category            : chr  "001" "001" "001" "001" ...
    ##  $ subcategory         : chr  NA "0016" "0016" "0016" ...
    ##  $ subclassification   : chr  NA "00160" "00160" "00160" ...
    ##  $ subsubclassification: chr  NA "001607" "001607" "001607" ...
    ##  $ extension           : chr  NA "0016070" "0016071" "0016072" ...

The additional columns, in order of hierarchy, are:

- chapter
- subchapter
- category
- subcategory
- subclassification
- subsubclassification
- extension

To keep the install size of `medicalcoder` under the size limits for CRAN, the stored data is structured in a way that several joins and other operations are need to have a data set that is end user friendly. Several data sets are generated and cached when the namespace is loaded.

# `lookup_icd_codes()`

A related function, `lookup_icd_codes()`, allows the user to look up specific ICD codes. The return is a `data.frame`. The columns report the input code, if it was matched as a full code (with an applicable decimal point) or a compact code (applicable decimal point omitted) along with the ICD version ,type, and when the code was assignable.

    codes <- c("0011", "7329", "732", "73291", "not a code", "001.1", "A9248", "A924", "Z00")
    knitr::kable(lookup_icd_codes(codes), row.names = FALSE)

input_code | match_type | icdv | dx | full_code | code | src | known_start | known_end | assignable_start | assignable_end
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
0011 | compact_code | 9 | 0 | 00.11 | 0011 | cms | 2003 | 2015 | 2003 | 2015
0011 | compact_code | 9 | 1 | 001.1 | 0011 | cms | 1997 | 2015 | 1997 | 2015
7329 | compact_code | 9 | 1 | 732.9 | 7329 | cms | 1997 | 2015 | 1997 | 2015
732 | compact_code | 9 | 0 | 73.2 | 732 | cms | 1997 | 2015 |  |
732 | full_code | 9 | 1 | 732 | 732 | cms | 1997 | 2015 |  |
73291 |  |  |  |  |  |  |  |  |  |
not a code |  |  |  |  |  |  |  |  |  |
001.1 | full_code | 9 | 1 | 001.1 | 0011 | cms | 1997 | 2015 | 1997 | 2015
A9248 |  |  |  |  |  |  |  |  |  |
A924 | compact_code | 10 | 1 | A92.4 | A924 | who | 2008 | 2019 | 2008 | 2019
A924 | compact_code | 10 | 1 | A92.4 | A924 | cdc | 2001 | 2025 | 2001 | 2025
A924 | compact_code | 10 | 1 | A92.4 | A924 | cms | 2014 | 2026 | 2014 | 2026
Z00 | full_code | 10 | 1 | Z00 | Z00 | cms | 2014 | 2026 |  |
Z00 | full_code | 10 | 1 | Z00 | Z00 | who | 2008 | 2019 |  |

It is possible to restrict the look up to just full or compact codes. The default, as shown above, is to consider full and compact codes. Set `full.codes = FALSE` so only compact codes are considered.

    knitr::kable(
      lookup_icd_codes(codes, full.codes = FALSE),
      row.names = FALSE
    )

input_code | match_type | icdv | dx | full_code | code | src | known_start | known_end | assignable_start | assignable_end
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
0011 | compact_code | 9 | 0 | 00.11 | 0011 | cms | 2003 | 2015 | 2003 | 2015
0011 | compact_code | 9 | 1 | 001.1 | 0011 | cms | 1997 | 2015 | 1997 | 2015
7329 | compact_code | 9 | 1 | 732.9 | 7329 | cms | 1997 | 2015 | 1997 | 2015
732 | compact_code | 9 | 0 | 73.2 | 732 | cms | 1997 | 2015 |  |
732 | compact_code | 9 | 1 | 732 | 732 | cms | 1997 | 2015 |  |
73291 |  |  |  |  |  |  |  |  |  |
not a code |  |  |  |  |  |  |  |  |  |
001.1 |  |  |  |  |  |  |  |  |  |
A9248 |  |  |  |  |  |  |  |  |  |
A924 | compact_code | 10 | 1 | A92.4 | A924 | cdc | 2001 | 2025 | 2001 | 2025
A924 | compact_code | 10 | 1 | A92.4 | A924 | cms | 2014 | 2026 | 2014 | 2026
A924 | compact_code | 10 | 1 | A92.4 | A924 | who | 2008 | 2019 | 2008 | 2019
Z00 | compact_code | 10 | 1 | Z00 | Z00 | who | 2008 | 2019 |  |
Z00 | compact_code | 10 | 1 | Z00 | Z00 | cms | 2014 | 2026 |  |

And set `compact.codes = FALSE` to only consider full codes.

    knitr::kable(
      lookup_icd_codes(codes, compact.codes = FALSE),
      row.names = FALSE
    )

input_code | match_type | icdv | dx | full_code | code | src | known_start | known_end | assignable_start | assignable_end
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
0011 |  |  |  |  |  |  |  |  |  |
7329 |  |  |  |  |  |  |  |  |  |
732 | full_code | 9 | 1 | 732 | 732 | cms | 1997 | 2015 |  |
73291 |  |  |  |  |  |  |  |  |  |
not a code |  |  |  |  |  |  |  |  |  |
001.1 | full_code | 9 | 1 | 001.1 | 0011 | cms | 1997 | 2015 | 1997 | 2015
A9248 |  |  |  |  |  |  |  |  |  |
A924 |  |  |  |  |  |  |  |  |  |
Z00 | full_code | 10 | 1 | Z00 | Z00 | who | 2008 | 2019 |  |
Z00 | full_code | 10 | 1 | Z00 | Z00 | cms | 2014 | 2026 |  |

By default, `lookup_icd_codes()` considers the input to be a string and a direct match to the internal lookup table is made.

`lookup_icd_codes()` can also accept regular expressions. By providing a vector of regular expression patterns for the codes (passed to `grep()`)

    knitr::kable(
      lookup_icd_codes(x = "^C84\\.6[0-1A-Z]", regex = TRUE),
      row.names = FALSE
    )

input_regex | match_type | icdv | dx | full_code | code | src | known_start | known_end | assignable_start | assignable_end
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
^C84.6[0-1A-Z] | full_code | 10 | 1 | C84.60 | C8460 | cms | 2014 | 2026 | 2014 | 2026
^C84.6[0-1A-Z] | full_code | 10 | 1 | C84.61 | C8461 | cms | 2014 | 2026 | 2014 | 2026
^C84.6[0-1A-Z] | full_code | 10 | 1 | C84.6A | C846A | cms | 2025 | 2026 | 2025 | 2026

# `is_icd()`

By convention, ICD codes are generally reported without decimal points. Under this convention discriminating between ICD-9 and ICD-10, and between diagnostic and procedure codes can be difficult.

Is “7993” a valid code? It is not a valid ICD-10 code as a four digit code could not be an ICD-10 procedure code, and all ICD-10 diagnostic codes start with a letter, not a number. So this string could only be a ICD-9 code. It is a valid ICD-9 diagnostic code, and a valid ICD-9 procedure code.

    is_icd(x = "7993")
    ## Warning: Input '7993' is a: ICD 9 dx compact; ICD 9 pr compact
    ## [1] TRUE
    is_icd(x = "7993", icdv =  9, dx = 1)
    ## [1] TRUE
    is_icd(x = "7993", icdv =  9, dx = 0)
    ## [1] TRUE
    is_icd(x = "7993", icdv = 10, dx = 1)
    ## [1] FALSE
    is_icd(x = "7993", icdv = 10, dx = 0)
    ## [1] FALSE
    lookup_icd_codes("7993")
    ##   input_code   match_type icdv dx full_code code src known_start known_end
    ## 1       7993 compact_code    9  1     799.3 7993 cms        1997      2015
    ## 2       7993 compact_code    9  0     79.93 7993 cms        1997      2015
    ##   assignable_start assignable_end
    ## 1             1997           2015
    ## 2             1997           2015

A vector of possible codes:

    x <- c("7993", "A924", "7993", "A924", "no", "A92", "516", "5163", "51631", "A00")
    is_icd(x)
    ## Warning: Input '7993' is a: ICD 9 dx compact; ICD 9 pr compact
    ## Warning: Input '5163' is a: ICD 9 dx compact; ICD 9 pr compact
    ##  [1]  TRUE  TRUE  TRUE  TRUE FALSE FALSE FALSE  TRUE  TRUE FALSE

If you have codes with decimal points then discriminating between ICD-9 diagnostic and procedure codes can be done.

    x <- c("7993",  # valid dx and pr code
           ".7993", # not a valid code
           "7.993", # not a valid code
           "79.93", # invalid dx code; valid pr code
           "799.3", # valid dx code; invalid pr code
           "7993.") # not a valid code
    data.frame(x = x,
               icd9_dx = is_icd(x, icdv = 9, dx = 1, warn.ambiguous = FALSE),
               icd9_pr = is_icd(x, icdv = 9, dx = 0, warn.ambiguous = FALSE))
    ##       x icd9_dx icd9_pr
    ## 1  7993    TRUE    TRUE
    ## 2 .7993   FALSE   FALSE
    ## 3 7.993   FALSE   FALSE
    ## 4 79.93   FALSE    TRUE
    ## 5 799.3    TRUE   FALSE
    ## 6 7993.   FALSE   FALSE

### Assignable codes

Ideally, codes are reported with the greatest level of detail. While there is always a chance for incomplete coding, it is possible that an assignable code in one year becomes a header code in a subsequent year. Let’s look at the ICD-9 DX code 516.3 and five digit codes 516.30 through 516.39 (not all of these are valid, as we’ll see in the examples.)

Given the default settings, we have the following results for testing if these strings are valid ICD-9 dx codes.

By default, if no year is provided in the `is_icd()` call then return will be `TRUE` if the code was ever assignable.

    x <- paste0("516.3", c("", 0:9))
    tab <-
      data.frame(
        code       = x,
        default    = is_icd(x, icdv = 9, dx = 1),
        assignable_1997 = is_icd(x, src = "cms", icdv = 9, dx = 1, year = 1997),
        assignable_2010 = is_icd(x, src = "cms", icdv = 9, dx = 1, year = 2010),
        assignable_2011 = is_icd(x, src = "cms", icdv = 9, dx = 1, year = 2011),
        assignable_2012 = is_icd(x, src = "cms", icdv = 9, dx = 1, year = 2012),
        assignable_2013 = is_icd(x, src = "cms", icdv = 9, dx = 1, year = 2013),
        assignable_2016 = is_icd(x, src = "cms", icdv = 9, dx = 1, year = 2016),
        assignable_ever = is_icd(x, src = "cms", icdv = 9, dx = 1, ever.assignable = TRUE)
      )
    ## Warning: The combination of `icdv` = 9; `dx` = 1; and `src` = cms; has ICD
    ## codes with a max known_end year of 2015. The input of `year` = 2016 results in
    ## no possible positive match.
    knitr::kable(tab)

code | default | assignable_1997 | assignable_2010 | assignable_2011 | assignable_2012 | assignable_2013 | assignable_2016 | assignable_ever
--- | --- | --- | --- | --- | --- | --- | --- | ---
516.3 | TRUE | TRUE | TRUE | TRUE | FALSE | FALSE | FALSE | TRUE
516.30 | TRUE | FALSE | FALSE | FALSE | TRUE | TRUE | FALSE | TRUE
516.31 | TRUE | FALSE | FALSE | FALSE | TRUE | TRUE | FALSE | TRUE
516.32 | TRUE | FALSE | FALSE | FALSE | TRUE | TRUE | FALSE | TRUE
516.33 | TRUE | FALSE | FALSE | FALSE | TRUE | TRUE | FALSE | TRUE
516.34 | TRUE | FALSE | FALSE | FALSE | TRUE | TRUE | FALSE | TRUE
516.35 | TRUE | FALSE | FALSE | FALSE | TRUE | TRUE | FALSE | TRUE
516.36 | TRUE | FALSE | FALSE | FALSE | TRUE | TRUE | FALSE | TRUE
516.37 | TRUE | FALSE | FALSE | FALSE | TRUE | TRUE | FALSE | TRUE
516.38 | FALSE | FALSE | FALSE | FALSE | FALSE | FALSE | FALSE | FALSE
516.39 | FALSE | FALSE | FALSE | FALSE | FALSE | FALSE | FALSE | FALSE

Similar information can be quickly and easily retrieved via `lookup_icd_codes()`.

    knitr::kable(lookup_icd_codes(x))

input_code | match_type | icdv | dx | full_code | code | src | known_start | known_end | assignable_start | assignable_end
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
516.3 | full_code | 9 | 1 | 516.3 | 5163 | cms | 1997 | 2015 | 1997 | 2011
516.30 | full_code | 9 | 1 | 516.30 | 51630 | cms | 2012 | 2015 | 2012 | 2015
516.31 | full_code | 9 | 1 | 516.31 | 51631 | cms | 2012 | 2015 | 2012 | 2015
516.32 | full_code | 9 | 1 | 516.32 | 51632 | cms | 2012 | 2015 | 2012 | 2015
516.33 | full_code | 9 | 1 | 516.33 | 51633 | cms | 2012 | 2015 | 2012 | 2015
516.34 | full_code | 9 | 1 | 516.34 | 51634 | cms | 2012 | 2015 | 2012 | 2015
516.35 | full_code | 9 | 1 | 516.35 | 51635 | cms | 2012 | 2015 | 2012 | 2015
516.36 | full_code | 9 | 1 | 516.36 | 51636 | cms | 2012 | 2015 | 2012 | 2015
516.37 | full_code | 9 | 1 | 516.37 | 51637 | cms | 2012 | 2015 | 2012 | 2015
516.38 |  |  |  |  |  |  |  |  |  |
516.39 |  |  |  |  |  |  |  |  |  |

For fiscal years 1997 through 2011 the code 516.3 was assignable. In 2012 516.3 was not assignable due to the introduction of the five digit codes 516.30, 516.31, 516.32, 516.33, 516.34, 516.35, 516.36, and 516.37. Codes 526.38 and 516.39 were never in the ICD-9-CM standard. When looking at retrospective data over several years the use of the `ever.assignable` argument will simplify the testing for valid codes.

### Header codes

There is also an option to considering header codes to be valid. As seen below, the code “516” is a header, it was never assignable in ICD-9-CM. By setting `headerok = TRUE` “516” will be flagged as a valid code. A ICD-10 header “A00” will be FALSE in the following checks of ICD-9 codes.

    x <- c("516", "5163", "51631", "A00")
    tab <-
      data.frame(
        code     = x,
        default  = is_icd(x, icdv = 9, dx = 1, src = "cms", headerok = FALSE, ever.assignable = FALSE, warn.ambiguous = FALSE),
        ever     = is_icd(x, icdv = 9, dx = 1, src = "cms", headerok = FALSE, ever.assignable = TRUE,  warn.ambiguous = FALSE),
        headerok = is_icd(x, icdv = 9, dx = 1, src = "cms", headerok = TRUE,                           warn.ambiguous = FALSE)
      )
    knitr::kable(tab)

code | default | ever | headerok
--- | --- | --- | ---
516 | FALSE | FALSE | TRUE
5163 | FALSE | TRUE | TRUE
51631 | TRUE | TRUE | TRUE
A00 | FALSE | FALSE | FALSE

A more complex situation is ICD-9-CM code 719.7 and the five digit codes 719.70, 719.75, 719.76, 719.77, 719.78, and 719.79. The five digit codes were assignable codes through FY 2004. Starting in FY 2004 the five digit codes were removed from the standard and the four digit code became assignable. This is a rare example of a header code becoming assignable.

    x <- paste0("719.7", c("", "0", 5:9))
    tab <-
      data.frame(
        code            = x,
        default         = is_icd(x, src = "cms", icdv = 9, dx = 1),
        assignable_2002 = is_icd(x, src = "cms", icdv = 9, dx = 1, year = 2002),
        assignable_2003 = is_icd(x, src = "cms", icdv = 9, dx = 1, year = 2003),
        assignable_2004 = is_icd(x, src = "cms", icdv = 9, dx = 1, year = 2004),
        assignable_2005 = is_icd(x, src = "cms", icdv = 9, dx = 1, year = 2005),
        assignable_ever = is_icd(x, src = "cms", icdv = 9, dx = 1, ever.assignable = TRUE)
      )
    knitr::kable(tab)

code | default | assignable_2002 | assignable_2003 | assignable_2004 | assignable_2005 | assignable_ever
--- | --- | --- | --- | --- | --- | ---
719.7 | TRUE | FALSE | FALSE | TRUE | TRUE | TRUE
719.70 | TRUE | TRUE | TRUE | FALSE | FALSE | TRUE
719.75 | TRUE | TRUE | TRUE | FALSE | FALSE | TRUE
719.76 | TRUE | TRUE | TRUE | FALSE | FALSE | TRUE
719.77 | TRUE | TRUE | TRUE | FALSE | FALSE | TRUE
719.78 | TRUE | TRUE | TRUE | FALSE | FALSE | TRUE
719.79 | TRUE | TRUE | TRUE | FALSE | FALSE | TRUE

# `icd_compact_to_full()`

To go from a full code to a compact code is simple, omit any decimal point in the string.

To go from a compact code to a full code requires knowing if the code is from version 9 or 10, and if it is a diagnostic or a procedure code. `icd_compact_to_full()` will format a string appropriately, within reason. This method only formats the strings and will not validate the return.

For example, the compact code “E1234” is in the format expected for a ICD-9 diagnostic code or ICD-10 diagnostic code. It could not be a procedure code as ICD-9 procedure codes are all numeric values and ICD-10 procedure codes are seven characters long. The actual code E1234 is not a valid ICD code. We use this string as an example.

    icd_compact_to_full("E1234", icdv =  9, dx = 1)
    ## [1] "E123.4"
    icd_compact_to_full("E1234", icdv = 10, dx = 1)
    ## [1] "E12.34"

    lookup_icd_codes(c("E1234", "E123.4", "E12.34"))[, c("input_code", "match_type")]
    ##   input_code match_type
    ## 3      E1234       <NA>
    ## 2     E123.4       <NA>
    ## 1     E12.34       <NA>

Notice that no change to the string is made when trying to convert to a full procedure code.

    icd_compact_to_full("E1234", icdv =  9, dx = 0)
    ## [1] "E1234"
    icd_compact_to_full("E1234", icdv = 10, dx = 0)
    ## [1] "E1234"

# General Notes on ICD Code Structure

All four sets of codes have a hierarchical structure. The first level of the hierarchy is the chapter which groups codes by disease category, body system, and/or condition. Following that are subchapters for all but the ICD-9 procedure codes. After the subchapter, depending on the ICD variant, are the category, subcategory, subclassification, subsubclassification, and extension.

## ICD-9 Diagnostic Codes

ICD-9 Diagnostic codes are organized by a hierarchy of five levels:

1. chapter,
2. subchapter,
3. category,
4. subcategory, and
5. subclassification.

ICD-9 diagnostic codes are three to five digits, not counting a decimal point, numeric or alpha numeric strings. The first three digits are the category with numeric code 000 through 999 (leading zeros are part of the numeric code), or V00-V99, or E000-E999. When the category does not provide sufficient detail, a fourth numeric digit, separated from the category by a decimal point, is used. Lastly, when the subcategory is insufficient detail, then a fifth numeric digit is used, save for the E categories.

## ICD-9 Procedure Codes

ICD-9 Procedure codes are organized by a hierarchy of four levels:

1. chapter,
2. category,
3. subcategory, and
4. subclassification.

The codes are numeric strings of four digits with a decimal point between the second and third digits. The first two digits are the category, the third digit is the subcategory, and the fourth digit is the subclassification.

## ICD-10 Diagnostic Codes

ICD-10 diagnostic codes are up to seven alphanumeric codes with a hierarchy of

1. chapter,
2. subchapter,
3. category,
4. subcategory,
5. subclassification,
6. subsubclassification, and
7. extension.

The category describes the general type of disease of injury, with the subcategory, subclassification and subsubclassification providing detail on the cause, manifestation, location, severity, and type of disease or injury. Finally, the extension specifies the type of encounter, i.e., initial or subsequent encounter, or sequela for encounters related to prior disease or injury.

## ICD-10 Procedure Codes

In general, ICD-10 procedure codes are seven digits. In medicalcoder, the three digit (chapter, subchapter, category) and the seven digit codes are in the data base.