Creating ADGA

Introduction

This article describes creating an ADGA ADaM dataset containing Geographic Atrophy (GA) data for ophthalmology endpoints. These endpoints are derived from Ophthalmic Examination (OE) records and include both observed and derived measurements for Geographic Atrophy. It is to be used in conjunction with the article on creating a BDS dataset from SDTM. As such, derivations and processes that are not specific to ADGA are absent, and the user is invited to consult the aforementioned article for guidance.

Note: All examples assume CDISC SDTM and/or ADaM format as input unless otherwise specified.

Dataset Contents

As the name ADGA implies, admiralophtha suggests to populate ADGA solely with GA records from the OE SDTM.

Acronyms

This vignette uses several ophthalmology-specific abbreviations related to Geographic Atrophy (GA) imaging. The table below summarizes the most commonly used terms.

Acronym	Definition
ADGA	Geographic Atrophy Analysis Dataset
FAF	Fundus Autofluorescence
GA	Geographic Atrophy
GAFLOC	GA Foveal Location
SGAFOCAL	Study Eye GA Lesion Focality
FGAFOCAL	Fellow Eye GA Lesion Focality
SAREAFAF	Study Eye GA Area Measured by FAF
FAREAFAF	Fellow Eye GA Area Measured by FAF
SSQRTFAF	Square Root Transformed Study Eye GA Area Measured by FAF
FSQRTFAF	Square Root Transformed Fellow Eye GA Area Measured by FAF

The parameter names used throughout this vignette follow the nomenclature adopted by admiralophtha examples, where the prefixes S and F denote the study eye and fellow eye, respectively.

Required Packages

The examples of this vignette require the following packages.

library(dplyr)
library(admiral)
library(pharmaversesdtm)
library(admiraldev)
library(admiralophtha)
library(stringr)

Programming Workflow

Initial Set Up of ADGA
Creating Square Root Transformed GA Area measured by FAF derived parameter
Assigning AVISIT/AVISITN
Deriving Worst Post-Baseline Assessment Flag
Further Derivations of Standard BDS Variables
Example Script

Initial set up of ADGA

As with all BDS ADaM datasets, one should start from the OE SDTM, where only records related to geographic atrophy are of interest. For the purposes of the next two sections, we shall be using the admiral OE and ADSL test data. We will also require a lookup table for the mapping of parameter codes. An SAREAFAF, FAREAFAF, SGAFOCAL and FGAFOCAL definition expression is created first - this expression can also be stored within another (sourced) program.

Note: to simulate an ophthalmology study, we add a randomly generated STUDYEYE variable to ADSL, but in practice STUDYEYE will already have been derived using derive_var_studyeye().

data("oe_ophtha")
data("admiral_adsl")

# Add STUDYEYE to ADSL to simulate an ophtha dataset
# Set seed to ensure reproducible STUDYEYE assignment across runs
set.seed(1234)

adsl <- admiral_adsl %>%
  as.data.frame() %>%
  mutate(STUDYEYE = sample(c("LEFT", "RIGHT"), n(), replace = TRUE)) %>%
  convert_blanks_to_na()

oe <- convert_blanks_to_na(oe_ophtha)

# ---- Lookup tables ----

# Assign PARAMCD, PARAM, and PARAMN
# nolint start
param_lookup <- tibble::tribble(
  ~OETESTCD, ~OECAT, ~OESCAT, ~AFEYE, ~PARAMCD, ~PARAM, ~PARAMN,
  "AREA", "OPHTHALMIC ASSESSMENTS", NA_character_, "Study Eye", "SAREAFAF", "Study Eye GA Area measured by FAF (mm2)", 1,
  "AREA", "OPHTHALMIC ASSESSMENTS", NA_character_, "Fellow Eye", "FAREAFAF", "Fellow Eye GA Area measured by FAF (mm2)", 2,
  "GAFLOC", "OPHTHALMIC ASSESSMENTS", NA_character_, "Study Eye", "SGAFOCAL", "Study Eye GA Lesion Focality", 3,
  "GAFLOC", "OPHTHALMIC ASSESSMENTS", NA_character_, "Fellow Eye", "FGAFOCAL", "Fellow Eye GA Lesion Focality", 4
)
# nolint end

Following this setup, the programmer can start constructing ADGA. The first step is to subset OE to only GA parameters like “AREA”, “GAFLOC” and merge with ADSL. This is required for two reasons: firstly, STUDYEYE is crucial in the mapping of AFEYE and PARAMCD’s. Secondly, the treatment start date (TRTSDT) is also a prerequisite for the derivation of variables such as Analysis Day (ADY).

# Get list of ADSL vars required for derivations
adsl_vars <- exprs(TRTSDT, TRTEDT, TRT01A, TRT01P, STUDYEYE)

adga <- oe %>%
  # Keep only GA related OE parameters
  filter(
    OETESTCD %in% c("AREA", "GAFLOC")
  ) %>%
  # Join ADSL with OE (need TRTSDT and STUDYEYE for ADY, AFEYE, and PARAMCD derivation)
  derive_vars_merged(
    dataset_add = adsl,
    new_vars = adsl_vars,
    by_vars = get_admiral_option("subject_keys")
  )

The next item of business is to derive AVAL,AVALC, AVALU, and DTYPE. In this example, derivation for AVALC is trivial, whereas for lesion focality, AVAL is derived using a combination of OE character standard results and decoded to 1 for S (single), and 2 for NS (multifocal). AFEYE is also created in this step using the function derive_var_afeye(). This variable identifies the analysis eye based on the OELOC, and OELAT information. In this example, "RETINA" is included in loc_vals because the GA assessments are recorded as retinal observations. Including both "EYE" and "RETINA" ensures that OELAT information is correctly carried forward for all ophthalmic assessments.

adga <- adga %>%
  # Calculate AVAL, AVALC, AVALU and DTYPE
  mutate(
    AVAL = case_when(
      OETESTCD == "GAFLOC" & OESTRESC == "S" ~ 1,
      OETESTCD == "GAFLOC" & OESTRESC == "NS" ~ 2,
      TRUE ~ OESTRESN
    ),
    AVALC = OESTRESC,
    AVALU = OESTRESU,
    DTYPE = NA_character_
  ) %>%
  # Derive AFEYE needed for PARAMCD derivation
  derive_var_afeye(loc_var = OELOC, lat_var = OELAT, loc_vals = c("EYE", "RETINA"))

Creating Square Root Transformed GA Area measured by FAF derived parameter

Next, PARAM, PARAMCD, and PARAMN are assigned using derive_vars_merged_lookup() from admiral together with the lookup table param_lookup generated above. Using a lookup table separates study-specific parameter metadata from the derivation code and provides an additional validation step. Since all records included in ADGA are expected to map to a defined analysis parameter, any not mapped records may indicate missing parameter metadata, unexpected source data, or an issue in an upstream derivation such as AFEYE. Therefore, check_not_mapped_type = “warning” is specified to issue a warning if any records are not mapped.

ADGA datasets often contain square root transformed GA records for FAF with units as millimeters (mm). In GA studies, square-root transformed lesion area is commonly used to reduce the dependence of lesion growth on baseline lesion size. Therefore, additional derived parameters are often included in ADGA representing the square root of the measured lesion area.

These derived parameters can be created using a single call to call_derivation() with derive_param_computed(). Using a single call is more efficient because the same grouping variables and source data are used for both study-eye and fellow-eye derivations. The parameter specifications differ only in the source parameter (SAREAFAF versus FAREAFAF), assigned parameter metadata (PARAM, PARAMCD, and PARAMN) and derived results (AVAL, and AVALC).

Note that the original lesion area measurements are recorded in square millimeters (mm²). Applying the square-root transformation changes the units from square millimeters (mm²) to millimeters (mm), since sqrt(mm²) = mm. Therefore, the derived parameters use mm as the analysis unit (AVALU).

Once these extra parameters are added, all the records that will be in the end dataset are now present, so day/date variables such as ADY and ADT can be derived.

adga <- adga %>%
  # Add PARAM, PARAMCD, PARAMN
  derive_vars_merged_lookup(
    dataset_add = param_lookup,
    new_vars = exprs(PARAM, PARAMCD, PARAMN),
    by_vars = exprs(OETESTCD, AFEYE)
  ) %>%
  # Add derived parameters for Square Root Transformed GA Area measured by FAF
  call_derivation(
    derivation = derive_param_computed,
    by_vars = c(
      get_admiral_option("subject_keys"),
      exprs(VISIT, VISITNUM, OEDY, OEDTC, AFEYE, !!!adsl_vars)
    ),
    variable_params = list(
      # Study eye
      params(
        # Users may need to update this code to identify the correct records to use.
        parameters = exprs(SESQRT = PARAMCD == "SAREAFAF"),
        set_values_to = exprs(
          PARAMCD = "SSQRTFAF",
          PARAM = "Study Eye Square Root Transformed GA Area measured by FAF (mm)",
          PARAMN = 5,
          AVAL = sqrt(AVAL.SESQRT),
          AVALC = as.character(AVAL),
          AVALU = "mm",
          DTYPE = "SQRT"
        )
      ),
      # Fellow eye
      params(
        # Users may need to update this code to identify the correct records to use.
        parameters = exprs(FESQRT = PARAMCD == "FAREAFAF"),
        set_values_to = exprs(
          PARAMCD = "FSQRTFAF",
          PARAM = "Fellow Eye Square Root Transformed GA Area measured by FAF (mm)",
          PARAMN = 6,
          AVAL = sqrt(AVAL.FESQRT),
          AVALC = as.character(AVAL),
          AVALU = "mm",
          DTYPE = "SQRT"
        )
      )
    )
  ) %>%
  # Calculate ADT, ADY
  derive_vars_dt(
    new_vars_prefix = "A",
    dtc = OEDTC,
    flag_imputation = "none"
  ) %>%
  derive_vars_dy(reference_date = TRTSDT, source_vars = exprs(ADT))
#> All `OETESTCD` and `AFEYE` are mapped.

Importantly, the above calls to derive_param_computed() list the SDTM variables VISIT, VISITNUM, OEDY and OEDTC as by_vars for the function. This is because they will be necessary to derive ADaM variables such as AVISIT and ADY in successive steps.

Assigning `AVISIT/AVISITN`

Moving forwards, AVISIT, AVISITN, and related timepoint variables can be derived soon after, though their derivation is generally study-specific. A simple option is included below; please consult the admiral BDS findings vignette for a more detailed discussion.

adga <- adga %>%
  # Derive visit info and BASETYPE
  mutate(
    ATPTN = OETPTNUM,
    ATPT = OETPT,
    AVISIT = case_when(
      str_detect(VISIT, "SCREEN") ~ "Screening",
      !is.na(VISIT) ~ str_to_title(VISIT),
      TRUE ~ NA_character_
    ),
    AVISITN = round(VISITNUM, 0),
    ABLFL = if_else(AVISIT == "Baseline", "Y", NA_character_)
    # In actual studies, ABLFL derivation will likely be more nuanced
    # and leverage derive_var_extreme_flag()
  )

Deriving Worst Post-Baseline Assessment Flag

For some analyses, it may be useful to identify the worst observed lesion size for each parameter across baseline and post-baseline assessments. The ADaM variable WORS01FL can be used for this purpose. In the example below, derive_var_extreme_flag() is used to flag the record with the highest observed value (AVAL) within each subject, parameter, and timepoint combination (PARAMCD, ATPT). Since disease progression in geographic atrophy is reflected by increasing lesion area, the highest observed FAF area (or square-root transformed FAF area) is considered the worst result.

The derivation is applied only to the quantitative FAF area parameters (SAREAFAF, FAREAFAF) and their square-root transformed counterparts (SSQRTFAF, FSQRTFAF). Categorical focality parameters (SGAFOCAL, FGAFOCAL) are excluded, as the concept of a worst numeric result is not applicable.

The order argument specifies that records are ranked first by AVAL and then by ADT. With mode = "last", the flag is assigned to the record with the largest value, and if multiple records share the same value, the latest assessment date is selected. If a parameter is instead interpreted such that lower values represent worse outcomes, mode = "first" can be used.

# WORS01FL: Flag worst result within a PARAMCD for baseline & post-baseline records
# If worst result is lowest result, change mode to "first"
adga <- adga %>%
  restrict_derivation(
    derivation = derive_var_extreme_flag,
    args = params(
      by_vars = c(get_admiral_option("subject_keys"), exprs(PARAMCD, ATPT)),
      order = exprs(AVAL, ADT),
      new_var = WORS01FL,
      mode = "last"
    ),
    filter = !is.na(AVISITN) & !(AVISITN) == 1 & PARAMCD %in% c("SSQRTFAF", "FSQRTFAF", "FAREAFAF", "SAREAFAF")
    # In actual studies, ONTRTFL == "Y" | ABLFL == "Y" will be used to consider baseline and post baseline assessments
  )

The resulting output is shown below (limited to the one patient and one parameter only):

Additionally, it should be noted that SGAFOCAL and FGAFOCAL represent categorical assessments of lesion focality. Although these parameters are mapped to numeric values in AVAL for consistency with ADaM conventions, deriving numeric change metrics such as CHG and PCHG is generally not meaningful. Therefore, BASE is not derived for these parameters.

However, baseline character values may still be clinically informative. For this reason, BASEC can be derived from AVALC for SGAFOCAL and FGAFOCAL, alongside the area-based FAF parameters. This can be achieved by using restrict_derivation() to selectively apply derive_var_base() based on parameter type. In the example below, BASE is derived only for parameters where numeric change-from-baseline analyses are appropriate, while SGAFOCAL and FGAFOCAL are excluded. Since BASE remains missing for SGAFOCAL and FGAFOCAL, subsequent calls to derive_var_chg() derive_var_pchg() will automatically leave CHG and PCHG missing for these parameters.

adga <- adga %>%
  # Calculate BASE (do not derive for GA Lesion Focality params)
  restrict_derivation(
    derivation = derive_var_base,
    args = params(
      by_vars = c(get_admiral_option("subject_keys"), exprs(PARAMCD, ATPT)),
      source_var = AVAL,
      new_var = BASE
    ),
    filter = !PARAMCD %in% c("SGAFOCAL", "FGAFOCAL")
  ) %>%
  # Calculate BASEC
  restrict_derivation(
    derivation = derive_var_base,
    args = params(
      by_vars = c(get_admiral_option("subject_keys"), exprs(PARAMCD, ATPT)),
      source_var = AVALC,
      new_var = BASEC
    ),
    filter = PARAMCD %in% c("SGAFOCAL", "FGAFOCAL", "SSQRTFAF", "FSQRTFAF", "FAREAFAF", "SAREAFAF")
  )

Further Derivations of Standard BDS Variables

The user is invited to consult the article on creating a BDS dataset from SDTM to learn how to add standard BDS variables to ADGA.

Example Script

ADaM	Sample Code
ADGA	`use_ad_template("adga", package = "admiralophtha")`