Data Format

FeRx reads data in NONMEM-compatible CSV format. This is the standard format used across population PK tools.

Required Columns

Column	Type	Description
`ID`	string/numeric	Subject identifier
`TIME`	numeric	Time relative to first dose
`DV`	numeric	Dependent variable (observed concentration)

Optional Standard Columns

Column	Type	Default	Description
`EVID`	integer	0	Event ID: 0 = observation, 1 = dose, 4 = reset + dose
`AMT`	numeric	0	Dose amount (only for `EVID=1` or `EVID=4`)
`CMT`	integer	1	Compartment number (1-indexed)
`RATE`	numeric	0	Infusion rate. 0 = bolus dose
`MDV`	integer	0	Missing DV flag. 1 = DV should be ignored
`II`	numeric	0	Interdose interval for repeated dosing
`SS`	integer	0	Steady-state flag. 1 = assume steady state
`CENS`	integer	0	Censoring flag. 1 = observation is below LLOQ; `DV` carries the LLOQ value. Paired with `bloq_method = m3` in `[fit_options]` to enable likelihood-based handling — see BLOQ example.

Covariate Columns

Any column not in the standard set above is automatically treated as a covariate. Covariate values are:

Time-constant: The first non-missing value for each subject is used
Time-varying: If values change over time for a subject, Last Observation Carried Forward (LOCF) is applied

Covariate names in the data file are matched case-insensitively to names used in [individual_parameters] expressions.

Event Types (EVID)

EVID	Meaning
0	Observation record. `DV` is used for estimation.
1	Dosing record. `AMT` is administered to compartment `CMT`.
4	Reset and dose. All compartment amounts are reset to zero before dosing.

Example Dataset

ID,TIME,DV,EVID,AMT,CMT,RATE,MDV,WT,CRCL
1,0,.,1,100,1,0,1,70,95
1,0.5,9.49,0,.,.,.,0,70,95
1,1,14.42,0,.,.,.,0,70,95
1,2,17.56,0,.,.,.,0,70,95
1,4,15.23,0,.,.,.,0,70,95
1,8,10.15,0,.,.,.,0,70,95
2,0,.,1,150,1,0,1,85,110
2,0.5,14.2,0,.,.,.,0,85,110
2,1,21.3,0,.,.,.,0,85,110

Key points:

Dose records (EVID=1) have MDV=1 and DV=. (missing)
Observation records (EVID=0) have MDV=0 and a valid DV
Covariates (WT, CRCL) are included as extra columns
Missing values can be represented as . or left empty

Infusion Doses

For IV infusions, set RATE to the infusion rate (amount per time unit):

ID,TIME,DV,EVID,AMT,CMT,RATE,MDV
1,0,.,1,500,1,50,1

This administers 500 units at a rate of 50 units/hour (duration = 10 hours).

Steady-State Dosing

For steady-state simulations, set SS=1 and II to the dosing interval:

ID,TIME,DV,EVID,AMT,CMT,SS,II,MDV
1,0,.,1,100,1,1,12,1
1,0.5,25.3,0,.,.,.,.,0

This assumes the subject has reached steady state with 100 units every 12 hours before the observation at TIME=0.5.

Multiple Doses

Multiple doses are supported as separate rows:

ID,TIME,DV,EVID,AMT,CMT,MDV
1,0,.,1,100,1,1
1,0.5,9.49,0,.,.,0
1,12,.,1,100,1,1
1,12.5,15.2,0,.,.,0
1,24,.,1,100,1,1
1,24.5,18.1,0,.,.,0

Column Name Case

Column names are case-insensitive. ID, Id, and id are all recognized. Covariate columns preserve their case as declared in the CSV header.

FeRx-NLME Documentation