Why Consider the Route?
An intravenous dose enters systemic circulation directly, whereas an intramuscular, subcutaneous, transdermal, or other extravascular dose must first be released from its administration site and reach the circulation. The amount that appears systemically can be less than the administered dose, and the routes can differ in the processes that cause that loss. For example, a drug delivered into muscle may remain at the injection site, while a transdermal dose may not fully leave its delivery system or cross the skin.
The relevant quantity is systemic availability: the fraction of the administered dose that reaches systemic circulation as the analyte being measured. An intravenous reference makes it possible to estimate this fraction from exposure, provided the reference and extravascular data satisfy the assumptions described below. The calculation is not specific to oral dosing; the route-specific interpretation is what changes.
Calculation Using an Intravenous Reference
Let \(AUC_r\) be the area under the plasma concentration–time curve after extravascular route \(r\), and let \(D_r\) be the administered dose by that route. Let \(AUC_{IV}\) and \(D_{IV}\) denote the corresponding quantities after intravenous administration. With complete exposure measured over the same relevant period, the estimate is
The result is a fraction; multiplying it by 100 expresses it as a percentage. If the doses are equal, the dose terms cancel and \(F_r=AUC_r/AUC_{IV}\). When doses differ, dose normalization is essential. For example, a larger AUC after a much larger extravascular dose does not by itself indicate greater availability.
A short derivation shows why the ratio estimates a fraction. Under linear pharmacokinetics, the total exposure for an administered dose \(D\) is the amount reaching systemic circulation divided by systemic clearance. If \(F_r\) is the fraction reaching circulation by route \(r\), then \(AUC_r=F_rD_r/CL\). For the intravenous reference, the fraction reaching circulation is 1, so \(AUC_{IV}=D_{IV}/CL\). Dividing the two dose-normalized exposures gives \(F_r\), because the common clearance cancels.
This cancellation is the key justification, not a guarantee that any two AUC values can be compared. The derivation requires comparable clearance and linear, dose-proportional exposure. The AUC must also represent the same analyte and account for the relevant concentration–time profile. If one route has delayed absorption, its concentrations may persist longer; AUC should still capture that exposure rather than truncate the profile prematurely.
What the Estimate Means for Different Routes
The estimate is route-specific because the processes between administration and systemic circulation depend on the route. For intramuscular dosing, incomplete release from the injection site, local degradation, or incomplete absorption can reduce systemic availability. Subcutaneous administration has its own site-dependent absorption and local loss. Transdermal administration may be limited by drug release from the delivery system, skin permeation, or loss at the skin surface. Other extravascular routes can introduce their own barriers and presystemic processes.
These are possible contributors to the observed estimate, not quantities that the AUC ratio separates. A value of \(F_r=0.72\), for example, indicates that systemic exposure per unit dose is 72% of the intravenous reference under the comparison assumptions. It does not, by itself, identify whether the remaining 28% failed to leave the administration site, was degraded, or was lost before reaching the measured systemic circulation.
This distinction is also why systemic availability is not synonymous with the fraction absorbed from a site. The estimate concerns appearance of the measured analyte in systemic circulation. It can reflect incomplete absorption as well as loss before systemic entry. If the measured analyte is a parent drug, conversion to a metabolite before systemic appearance of the parent can lower the parent’s systemic availability even if drug-related material was absorbed.
Worked Example: Intramuscular Dose With an Equal-Dose IV Reference
In a hypothetical comparison, an intravenous dose of 100 mg produces \(AUC_{IV}=500\) mg·h/L. An intramuscular dose of 100 mg produces \(AUC_{IM}=360\) mg·h/L. Estimate the absolute systemic availability after intramuscular dosing.
The doses are equal, but use the dose-normalized expression to make the comparison explicit:
Thus, the estimated intramuscular systemic availability is 0.720, or 72.0%. In this hypothetical comparison, each milligram administered intramuscularly yielded 72.0% of the measured systemic exposure per milligram administered intravenously. The calculation does not establish which muscle-site process accounts for the difference.
For interpretation, suppose the profiles were followed long enough to characterize the full exposure, the same analyte was measured, and dose-normalized exposure was proportional to dose over this range. Suppose also that systemic clearance was comparable between periods. Under these conditions, the AUC ratio is an estimate of route-specific systemic availability. If any of these conditions failed, the numerical ratio would not have that interpretation.
Worked Example: Unequal Doses After Subcutaneous and IV Administration
A hypothetical study gives a 60 mg intravenous dose with \(AUC_{IV}=240\) mg·h/L and a 90 mg subcutaneous dose with \(AUC_{SC}=270\) mg·h/L. Calculate subcutaneous absolute availability.
Because the doses differ, compare exposure per milligram rather than the raw AUC values:
The estimated subcutaneous availability is 0.750, or 75.0%. Although the subcutaneous AUC of 270 mg·h/L is greater than the intravenous AUC of 240 mg·h/L, the subcutaneous dose was also 50% larger. After dose normalization, exposure per milligram is lower for the subcutaneous route.
The interpretation assumes that exposure is proportional to dose over the 60–90 mg range and that the route comparison does not change systemic clearance. It also assumes that the measured AUCs adequately capture the subcutaneous absorption profile and the intravenous exposure. These are substantive conditions to check, not details that can be inferred from the arithmetic.
Worked Example: Transdermal Systemic Availability
In a hypothetical transdermal comparison, 200 mg of drug is applied using a delivery system, and the estimated AUC for the measured drug is 150 mg·h/L. A 50 mg intravenous dose gives an AUC of 250 mg·h/L. Estimate availability relative to IV.
The administered transdermal amount is the dose specified for this comparison. Using that amount and the IV dose:
The estimated systemic availability is 0.150, or 15.0% of the applied dose, relative to the intravenous reference. This does not mean that exactly 15% crossed the skin: the AUC comparison measures the fraction reaching systemic circulation as the measured analyte. The remaining applied dose may include drug that did not leave the system, did not cross the skin, or did not reach systemic circulation as the parent analyte.
A delayed transdermal profile makes adequate follow-up particularly important. If sampling ends while concentrations remain meaningfully above zero, the observed AUC may omit part of the exposure and bias the estimate downward. The calculation is interpretable only if the AUCs are sufficiently complete and the other comparison assumptions hold.
Assumptions, Diagnostics, and Failure Modes
Before interpreting a route-specific estimate, check the conditions that make the dose-normalized AUC ratio meaningful. In practical terms, the comparison should use:
- The same measured analyte. Comparing parent-drug exposure after one route with metabolite exposure after another does not estimate availability of a common analyte.
- Comparable systemic disposition. Clearance and other determinants of exposure after systemic entry should be the same or sufficiently comparable across the route conditions.
- Linear, dose-proportional exposure. The dose-normalized AUC should not change across the doses being compared. A dose–AUC plot or estimates at multiple doses can help reveal departures from proportionality.
- Adequately characterized AUC. Sampling should capture the relevant profile, including delayed absorption or a prolonged terminal tail. A route-specific delay should not be mistaken for absent exposure merely because follow-up was short.
- Comparable study conditions. Differences in subjects, time, co-medications, or physiological state can change systemic exposure independently of the route.
Useful diagnostics include inspecting concentration–time profiles for late concentrations, checking whether the terminal portion appears adequately characterized, and examining whether dose-normalized AUC is reasonably stable across doses. In a crossover comparison, the same participants receive both treatments, which can reduce between-person variability; the periods must still be sufficiently comparable. In a parallel comparison, differences in participants can affect exposure, so a route contrast needs an appropriate design and analysis. Neither design removes the need for the assumptions above.
A route may also produce a delayed or prolonged profile without changing total availability. If the full AUC is measured under linear pharmacokinetics and clearance is unchanged, the timing of systemic entry affects the shape of the concentration–time curve but not the dose-normalized total exposure implied by the amount reaching the circulation. By contrast, incomplete follow-up can make the measured AUC appear smaller for a slow route. That is a measurement limitation, not evidence by itself of lower true availability.
Absolute Availability and the Main Alternative
Absolute availability uses an intravenous reference and expresses extravascular systemic exposure relative to direct systemic entry. Relative bioavailability instead compares dose-normalized exposure between two non-intravenous treatments. A relative comparison can answer which of two extravascular routes or formulations produces more exposure per unit dose, but without an intravenous reference it does not establish the fraction of either dose reaching systemic circulation.
The absolute estimate is useful when the question is how much of an administered extravascular dose becomes systemically available, or when the route is being characterized against direct systemic delivery. Its limitations are equally important: it depends on a suitable IV reference, does not identify the mechanism of loss, and can be distorted by unequal systemic clearance, non-proportional exposure, or incomplete AUC measurement. It is an exposure-based estimate under stated conditions, not a direct measurement of drug remaining at an administration site.
Common Pitfall
Correction: Use dose-normalized AUC unless the doses are equal. For example, when the extravascular dose is four times the IV dose, the raw AUC ratio must be divided by four to obtain the dose-normalized comparison. Also, a delayed route requires adequate follow-up; a truncated AUC cannot be interpreted as the full route-specific systemic availability.
Check Your Understanding
Answer each question, then compare your reasoning with the solutions.
- A hypothetical 80 mg intramuscular dose gives \(AUC_{IM}=320\) mg·h/L. An 80 mg IV dose gives \(AUC_{IV}=400\) mg·h/L. Calculate \(F_{IM}\) and state its interpretation.
- A hypothetical 120 mg subcutaneous dose gives an AUC of 360 mg·h/L. A 60 mg IV dose gives an AUC of 240 mg·h/L. Calculate subcutaneous availability. Why is the raw AUC ratio misleading?
- Does a lower AUC after transdermal dosing establish that little drug crossed the skin? Explain what the systemic availability estimate can and cannot identify.
- A slow-release extravascular profile is still measurable when sampling stops. What is the likely concern for the AUC-based availability estimate?
- Name two assumptions that make the IV-reference ratio interpretable as absolute systemic availability.
Solutions. (1) \(F_{IM}=(320/80)/(400/80)=4.00/5.00=0.800\), or 80.0%. Under the comparison assumptions, exposure per milligram after intramuscular dosing is 80.0% of exposure per milligram after IV dosing. (2) \(F_{SC}=(360/120)/(240/60)=3.00/4.00=0.750\), or 75.0%. The raw AUC ratio is 1.50, but the subcutaneous dose is twice the IV dose, so the raw ratio does not compare exposure per unit dose. (3) No. The estimate concerns the measured analyte reaching systemic circulation; it does not separate failure to leave the delivery system, incomplete skin permeation, or loss before systemic entry. (4) The AUC may be incomplete and biased downward if follow-up misses the delayed exposure. (5) Examples include dose-proportional exposure, comparable systemic clearance, measurement of the same analyte, and sufficiently complete AUCs.