Volume of distribution units

[js3110]

Hi! At the moment, the Vz and parameter units are just Cl / Lambda z, which for the dataset I am using comes to mg/(ng/mL) .
Seeing as Vz is a volume, is there a way to get PKNCA to simplify/convert the result to ml?

[billdenney]

You can use the conversions argument to pknca_units_table() to convert units to mL. Documentation for specifics are here: https://humanpred.github.io/pknca/articles/v07-unit-conversion.html#prepare-a-unit-assignment-and-conversion-table

FYI, the reason that it doesn't do automatic unit simplification like this is because some units (notably dose-normalized units) are not the desired units if automatic simplification occurs. I have debated some ways to automate this simplification, and would be interested to hear ideas.

[js3110]

Hi, I see your point. I would say that for volume specific units you could try automated simplification, since these are the only parameters where there seems to be a disconnect between the units and desired units. Here is an idea for an update to the pknca_units_table_conc_dose function:

pknca_units_table_conc_dose <- function(concu, doseu) {
  if (useless(concu) || useless(doseu)) {
    conc_dosenorm <- NA_character_
    volume <- NA_character_
  } else {
    conc_dosenorm <- sprintf(
      "%s/%s",
      pknca_units_add_paren(concu), 
      pknca_units_add_paren(doseu)
    )
    
    # Attempt to extract volume unit from the concentration unit
    concu_parts <- strsplit(concu, "/", fixed = TRUE)[[1]]
    if (length(concu_parts) == 2) {
      volume <- concu_parts[2]
    } else {
      # Fallback to the original compound unit calculation
      volume <- sprintf(
        "%s/%s",
        pknca_units_add_paren(doseu),
        pknca_units_add_paren(concu)
      )
    }
  }
  
  rbind(
    data.frame(
      PPORRESU = conc_dosenorm,
      PPTESTCD = pknca_find_units_param(unit_type = "conc_dosenorm"),
      stringsAsFactors = FALSE
    ),
    data.frame(
      PPORRESU = volume,
      PPTESTCD = pknca_find_units_param(unit_type = "volume"),
      stringsAsFactors = FALSE
    )
  )
}

Since all possible volume units are single strings (no /), this way you extract the volume unit from concentration and that is returned as the volume unit, instead of the compound version.
The only thing is idk if returning this unit also includes the conversion required, or if you would also have to update the pk.calc.vz funciton to convert it appropriately. Let me know what you think!