Pregabalin — steady-state calculator

This document explains what happens "under the hood" of the calculator: what assumptions the model makes, where the equations come from and — most importantly for everyday practice — how to interpret the numbers the calculator displays. It is written for a physician who does not specialise in pharmacokinetics professionally. Every equation is accompanied by a clinical translation: what each symbol means and why the result matters at the bedside.

Before we proceed, one overriding caveat that applies throughout this document. The calculator is an educational tool and does not replace clinical judgment or individual therapeutic drug monitoring (TDM). It shows how pregabalin concentrations would behave in a typical patient with given parameters — not in the specific patient sitting in your office. That distinction is at the heart of everything described below, and we will return to it repeatedly.

The guide is divided into four parts. Part I explains the model and formulas. Part II covers each of the eight output parameters. Part III addresses the fluctuation alert and the effect of dosing frequency. Part IV describes special patient populations and the limits beyond which the calculator is no longer reliable.

The one-compartment model — why we treat the body as a single tank

The calculator is based on a one-compartment model (one-compartment model) with first-order kinetics (first-order kinetics). This sounds abstract, but the idea is simple. The model imagines the entire organism as a single fluid compartment in which the drug distributes instantly and uniformly. The concentration measured in plasma represents the concentration "in the whole tank".

Why such a simplified picture? Because pregabalin actually behaves in the body in an exceptionally "predictable" way, which makes the one-compartment model appropriate. Pregabalin does not bind to plasma proteins, its metabolism is below 1% of the dose, and nearly all the dose is excreted unchanged by the kidneys.¹ There is no complex distribution to multiple tissues with different affinities, no active metabolites, no hepatic interactions. This is a rare example of a drug whose fate in the body can be described by a single simple relationship.

First-order kinetics means that the rate of elimination is proportional to the current concentration — the more drug in the blood, the faster it is removed, always at a constant percentage per unit time. This is confirmed by pregabalin's linear pharmacokinetics: its bioavailability remains ≥ 90% regardless of dose across the 75–900 mg/d range.¹ Doubling the dose doubles the concentration — without the pitfalls of saturable absorption or elimination mechanisms.

Absorption: absorption rate constant and bioavailability

Bioavailability F = 0.90 means that 90% of the orally administered dose reaches the systemic circulation. This value was adopted in the model based on population data, consistent with the observed bioavailability of ≥ 90% for pregabalin.¹ In practice: from a 150 mg tablet, approximately 135 mg of pharmacologically active drug reaches the bloodstream.

The absorption rate constant k_a = 2.1 h⁻¹ describes how quickly the drug passes from the gut into the blood. The higher the value, the faster the absorption. This particular value corresponds to a time to peak concentration (T_max) of approximately 1 hour fasted¹ — pregabalin is absorbed rapidly. One nuance not captured in the model is worth noting: food delays T_max to 2–3 hours and reduces peak concentration by 25–31%, but does not change total exposure (AUC), so no dose adjustment relative to meals is required.¹ The calculator assumes fasted administration.

Apparent clearance CL/F — the engine of the entire model

If any formula deserves to be called the heart of the calculator, it is this one. Apparent clearance (apparent clearance, CL/F) is the volume of plasma completely "cleared" of drug per unit time. It determines how quickly the body eliminates pregabalin and, therefore, what concentrations will be established at a given dosing regimen.

where m is body weight in kilograms and CrCl is creatinine clearance in mL/min. Let us unpack this formula into three clinical intuitions.

First, clearance increases linearly with body weight — a larger patient has a proportionally higher clearance. Second, and most importantly, clearance is directly proportional to creatinine clearance. The term (CrCl/90) is simply the patient's renal function referenced to a normal value of 90 mL/min. In a patient with normal renal function (CrCl = 90) this fraction equals 1 and does not modify the result. In a patient with CrCl = 45 mL/min the fraction drops to 0.5, which halves clearance — the drug is eliminated twice as slowly and accumulates accordingly.

This linear relationship is not an arbitrary choice by the calculator's designer. It follows from physiology: since pregabalin is excreted almost exclusively by the kidneys, its clearance must track glomerular filtration. Randinitis et al. (2003) demonstrated that pregabalin clearance is proportional to CrCl with a very strong correlation (r² = 0.83), and the regression equation had the form CL_oral = 0.56 × CrCl + 4.7 mL/min — with the intercept not differing significantly from zero (p > 0.05), providing additional confirmation of negligible systemic metabolism.² Krasowski (2010) independently confirms that pregabalin clearance corresponds to the rate of glomerular filtration (GFR), and that approximately 98% of the dose is excreted in urine unchanged.³ The formula in the calculator is a practical implementation of exactly this observation — which is why apparent clearance in the results panel is annotated "≈ creatinine clearance".

Elimination rate constant and half-life

Two further quantities follow from clearance and volume of distribution.

Volume of distribution V_d = 0.5 L/kg is the theoretical volume in which the drug would be dissolved if its concentration everywhere equalled the plasma concentration. For pregabalin this amounts to half a litre per kilogram of body weight — a value derived from population data.¹ Importantly, the volume of distribution of pregabalin remains constant regardless of renal function², so the entire impact of renal impairment is channelled through clearance, not distribution.

The elimination rate constant k_e follows from a simple ratio:

The higher the clearance relative to the volume of distribution, the faster the elimination. From this constant the calculator derives the half-life (t½) — the time it takes for the drug concentration to fall by half. In a patient with normal renal function, the mean half-life of pregabalin is approximately 6.3 hours.¹ It is t½ that governs the rate of approach to steady state, as we will see shortly.

The Bateman function and the superposition principle — the origin of the concentration curve

The concentration profile graph is generated by a mathematical equation known as the Bateman function. It describes the shape of the concentration-time curve after oral dosing: first a rapid rise (absorption phase), reaching a peak, then a gradual decline (elimination phase). This characteristic hump is the result of two opposing processes — drug flowing in from the gut and drug being removed by the kidneys.

At steady state, when the patient takes the drug regularly, the calculator applies the superposition principle. The idea is as follows: each successive dose creates its own Bateman curve, and the actual concentration at any moment is the sum of contributions from all previous doses that have not yet been fully eliminated. The morning dose still "contributes" to the concentration after the afternoon dose. The full equation takes the form:

where τ (tau) is the dosing interval and the symbol Σ (sigma) denotes summation of contributions from successive doses. The physician does not need to calculate this by hand — what matters is the interpretation: the shape of the curve and the height of the plateau depend on the interplay between absorption rate, elimination rate and dosing frequency. This is why the same drug given twice daily will produce a different profile than given four times daily at the same total daily dose — we will return to this in Part III.

Steady state

Steady state (SS) is the point at which the amount of drug taken in balances the amount eliminated, and concentrations day by day stop rising and settle into a repeatable daily rhythm. All parameters in the results panel labelled with the "SS" subscript refer to this stabilised state.

The practical rule is that steady state is reached after approximately five half-lives. At normal renal function with t½ ≈ 6.3 h this means 24–48 hours.¹ The calculator panel states this explicitly next to the half-life: "SS ≈ 1.3 days (5 × t½)". This has direct clinical significance: after a dose change, the new concentration will not be established until that time has elapsed, and in a patient with renal impairment — when t½ extends to several dozen hours — correspondingly later.

The calculator's main panel displays eight numbers. Below we discuss each of them: what it means, how to read it and when it deserves special attention. All values refer to steady state in a typical patient with the given parameters. For orientation, the table below lists all eight parameters; a detailed clinical discussion of each follows.

C_max,SS — peak concentration

C_max,SS is the highest concentration reached during the day at steady state, reported together with the time of its occurrence. Clinically it corresponds to "how high" the drug concentration goes shortly after dose absorption.

Peak concentration is primarily relevant for safety and tolerability. It is usually around the peak that patients most strongly experience dose-dependent effects — drowsiness, dizziness, impaired coordination. If a patient reports pronounced adverse effects shortly after taking the drug, a high C_max,SS value in the calculator may illustrate why this is happening and suggest considering splitting the daily dose into smaller portions. Bear in mind, however, that this is an illustration of a mechanism, not a diagnosis. A real patient may tolerate higher concentrations than the model predicts — or vice versa.

C_min,SS — minimum concentration (trough)

C_min,SS is the lowest concentration in the daily cycle, occurring just before the next dose — hence called the "trough". It is also reported with its time of occurrence.

The trough speaks to the risk of subtherapeutic exposure at the end of the dosing interval. If C_min,SS falls clearly below the lower boundary of the range (2 µg/mL), it means that for part of the day — just before the next dose — drug concentration may be too low to maintain the effect. In a patient with neuropathic pain this may correspond to recurrence of symptoms in the early morning. A clinical solution is often to shorten the dosing interval, which raises the trough at the cost of a smaller amplitude of fluctuation.

C_avg,SS — mean concentration

C_avg,SS is the average concentration over the course of the day, calculated as the area under the curve over the dosing interval divided by that interval (AUC_τ/τ). It can be thought of as the "centre of gravity" of the entire daily profile.

This is the most stable and representative single indicator of exposure. While the peak and trough describe the extremes, the mean concentration best correlates with the body's total drug exposure and, consequently, with the overall pharmacodynamic effect. To assess in a single number whether a patient is "approximately in range", C_avg,SS is the value to use.

AUC 24h — daily exposure

AUC 24h (area under the concentration-time curve over 24 hours) is a measure of the total systemic exposure to the drug over 24 hours, expressed in µg·h/mL. It is a mathematical summary of how much drug "passed through" the circulation during the day.

The key clinical intuition concerns its behaviour in renal impairment. Because AUC is inversely proportional to clearance, daily exposure increases as CrCl falls at the same dose. A patient with moderate chronic kidney disease (CKD), taking the same dose as a patient with normal renal function, will have substantially higher AUC — and this is the pharmacokinetic rationale for why doses are reduced as renal function deteriorates. The calculator allows this to be visualised: lowering the GFR slider at a fixed dose, one observes increasing AUC.

t½ — elimination half-life

t½ is the time it takes for drug concentration to fall by half. In the results panel this parameter serves a dual informational role: it describes the rate of elimination and — indirectly — the time to reach steady state (annotation "SS ≈ 5 × t½").

Clinically most important is the dramatic prolongation of half-life in renal impairment. For a patient with normal renal function the calculator uses a value of approximately 6 hours (consistent with the mean t½ = 6.3 h from the population pharmacokinetic analysis¹). As renal function deteriorates half-life increases markedly; data from Randinitis et al. (2003) from a single-dose study illustrate this eloquently: approximately 17 hours at CrCl 30–60 mL/min, approximately 25 hours at CrCl 15–29, and approximately 49 hours at CrCl < 15 mL/min.² It is worth noting that in the same single-dose study the terminal half-life in the group with normal renal function (CrCl > 60) was 9.1 h — the difference from the adopted 6.3 h value reflects different methodology (single-dose measurement versus steady state at multiple dosing) and is not a contradiction but a reflection of two different ways of estimating this parameter.

Prolongation of t½ in renal impairment means two things at once. First, in a patient with end-stage CKD the drug accumulates much more strongly. Second, steady state is reached only after many days, so the effects of any dose change manifest with a considerable delay — requiring patience and caution in interpretation.

CL/F — apparent clearance

CL/F (apparent clearance) was discussed in detail in Part I as the engine of the model. In the panel it is displayed in L/h with the annotation "≈ creatinine clearance", recalling its physiological significance.

In everyday interpretation, CL/F is the numerical representation of how efficiently the patient's kidneys eliminate the drug. Its correlation with creatinine clearance is the basis for all dose adjustments in renal impairment. The practical rule derived from the studies reads: for every 50% reduction in CrCl below 60 mL/min, the total daily dose is reduced by approximately 50%.² The aim of this rule is for pregabalin concentrations to differ by no more than twofold across the entire range of renal function — this explains why the dose is reduced, not just by how much.² The calculator allows this relationship to be traced directly — apparent clearance changes proportionally to the GFR slider setting.

Fluctuation F% — concentration swings

Fluctuation is a measure of the amplitude of concentration swings within the dosing cycle, calculated from the formula:

In other words, fluctuation describes how much concentration "jumps" between peak and trough relative to the mean value. Low fluctuation means a stable, flat profile; high fluctuation means large differences between the highest and lowest concentration during the day.

The strategy for reducing fluctuation is to administer the same total daily dose in more frequent, smaller portions. Pregabalin's short half-life makes it particularly susceptible to fluctuation at infrequent dosing — which is precisely why this parameter triggers the clinical alert described in Part III.

Time in therapeutic range

Time in therapeutic range is the fraction of the day during which pregabalin concentration falls within the accepted range of 2–8 µg/mL.⁴ The higher the value, the greater the proportion of the day the patient spends within the target concentration window.

High fluctuation alert

The calculator displays an orange warning panel titled "High concentration fluctuation" when F% exceeds a high threshold. The alert text is illustrative, for example: "105% at t½ 6.3 h — consider more frequent, smaller doses".

The alert is not a safety signal but an optimisation hint. It indicates that under the current regimen concentration swings are strong enough that the patient may experience both high peaks (worse tolerability) and deep troughs (weaker efficacy at the end of the interval). The suggested approach — more frequent, smaller doses while maintaining the same total daily dose — smooths the profile without changing total exposure. To be clear: the alert is based solely on the mathematics of concentration fluctuation, not on an assessment of any specific patient's condition, and does not replace the clinician's decision.

Effect of dosing frequency on the profile: BID, TID and QID

The same total daily dose distributed across different numbers of portions yields different concentration profiles — and this is one of the most instructive aspects of the calculator for learning by experiment. The mechanism follows directly from the superposition principle described in Part I.

The more portions per day, the lower the peaks, the higher the troughs and the smaller the fluctuation — at the cost of more frequent drug intake.

Pregabalin's short half-life (approximately 6.3 h at normal renal function)¹ means that the choice of dosing frequency has a clear, observable effect on the shape of the curve here. We encourage comparing these schedules in the calculator at an identical total daily dose — the variability of the curves is instructive.

Patients with CKD (stages 3–5)

This is the population for which the calculator is the most valuable tool, because it directly shows the consequences of impaired elimination. Lowering the GFR slider allows one to observe increasing AUC, extending half-life and the corresponding need for dose reduction. The progressively prolonging half-life as CrCl falls (tabulated in Part II) is a reminder that in these patients steady state is established slowly and the effects of any dose change manifest with delay. The model treats these values as averages — the real patient may deviate from the mean.

Dialysis patients — model limit

Obese patients — model limit

The calculator assumes a linear relationship of volume of distribution and clearance to body weight, adopting a uniform value of V_d = 0.5 L/kg.¹ In obese patients this assumption may be inaccurate, because the distribution of drug between adipose tissue and lean body mass is not a simple function of total body weight. Results for obese patients should therefore be interpreted with additional caution — the model has not been validated in this population.

This section collects in one place all the limitations that must be kept in mind with every use of the calculator. These are not formalities — they define the boundary between sensible and incorrect use of the tool.

Educational purpose. The calculator serves educational purposes and does not replace clinical judgment or individual therapeutic drug monitoring (TDM). All results are an illustration of pharmacokinetic mechanisms, not a recommendation for a specific patient.

Population model and inter-individual variability. The model assumes population-averaged parameters. According to the source data, inter-individual variability for peak concentration and exposure is 10–15% (coefficient of variation, CV)¹, and the model adopts a value of approximately 15%. This means that actual concentrations in an individual patient may deviate from the calculated values. Furthermore, the calculator's results do not account for pharmacokinetic interactions or haemodialysis.

Therapeutic range. The therapeutic range of pregabalin (2–8 µg/mL) is poorly established clinically — based on TDM data from epilepsy studies⁴ and not prospectively validated for pain or anxiety indications. The source literature itself notes that a definitive reference range for pregabalin has not been established.³

Haemodialysis. The model does not simulate concentration profiles in dialysis patients. Patients requiring haemodialysis require separate management — the calculator is not reliable for them.

1. Bockbrader HN et al. (2010). A comparison of the pharmacokinetics and pharmacodynamics of pregabalin and gabapentin. Clin Pharmacokinet, 49(10), 661–669.
2. Randinitis EJ et al. (2003). Pharmacokinetics of pregabalin in subjects with various degrees of renal function. J Clin Pharmacol, 43(3), 277–283.
3. Krasowski MD (2010). Therapeutic drug monitoring of the newer anti-epilepsy medications. Pharmaceuticals, 3(6), 1909–1935.
4. Johannessen Landmark C et al. (2020). Therapeutic drug monitoring of antiepileptic drugs. Expert Opin Drug Metab Toxicol. DOI: 10.1080/17425255.2020.1724956