Individual Parameters

The [individual_parameters] block defines how population parameters (theta), random effects (eta), and covariates combine to produce individual PK parameters.

Syntax

PARAM = expression

Each line assigns a PK parameter using an arithmetic expression that can reference:

• Theta parameters -- names defined in [parameters] (e.g., TVCL, TVV)
• Eta random effects -- names defined as omega parameters (e.g., ETA_CL, ETA_V)
• Covariates -- column names from the data file (e.g., WT, CRCL)
• Constants -- numeric literals (e.g., 70, 0.75)

Supported Operators and Functions

Common Parameterizations

Exponential (log-normal) random effects

The standard approach for PK parameters that must be positive:

[individual_parameters]
  CL = TVCL * exp(ETA_CL)
  V  = TVV  * exp(ETA_V)
  KA = TVKA * exp(ETA_KA)

Allometric scaling with covariates

[individual_parameters]
  CL = TVCL * (WT/70)^0.75 * exp(ETA_CL)
  V  = TVV  * (WT/70)^1.0  * exp(ETA_V)

Estimated covariate effects

Use additional theta parameters for covariate coefficients:

[parameters]
  theta TVCL(0.134, 0.001, 10.0)
  theta THETA_WT(0.75, 0.01, 2.0)
  theta THETA_CRCL(0.5, 0.01, 2.0)

[individual_parameters]
  CL = TVCL * (WT/70)^THETA_WT * (CRCL/100)^THETA_CRCL * exp(ETA_CL)

Covariate Detection

Any uppercase identifier in the expression that does not match a theta name or eta name is automatically treated as a covariate. The covariate value is read from the corresponding column in the data file.

For example, in CL = TVCL * (WT/70)^0.75 * exp(ETA_CL):

• TVCL matches a theta parameter
• ETA_CL matches an omega parameter
• WT matches neither, so it is treated as a covariate column

PK Parameter Names

The parameter names on the left side of each assignment must map to recognized PK parameter names:

For ODE models, the parameter names are user-defined and passed as a flat vector to the ODE right-hand side function.