Two-Level Elimination Rate Constant: Clinical Guide

Overview

Calculates a first-order elimination rate constant from two post-distribution concentrations and their sampling times.

Clinical Significance

This estimate is valid only when both samples lie on the same post-distribution, log-linear elimination phase and no additional dose occurred between them.

When to Use

• Estimate first-order elimination from two correctly timed drug concentrations.
• Use samples after distribution is complete.
• Confirm that no dose or extracorporeal clearance change occurred between samples.

How It Is Calculated

• k = ln(C1 ÷ C2) ÷ (t2 − t1).
• The calculator validates the required inputs before applying the equation and rounds only the displayed result.

Interpretation

• This estimate is valid only when both samples lie on the same post-distribution, log-linear elimination phase and no additional dose occurred between them.
• Review the component values and assumptions as well as the final result.

Worked Example

Enter clinically plausible values in the stated units. MedicalC applies: k = ln(C1 ÷ C2) ÷ (t2 − t1). Review the displayed result against the source data and the relevant clinical pathway.

Patient Considerations

• Confirm that the patient and clinical setting match the population or physiologic assumptions behind the equation.
• Consider whether treatment, organ function, pregnancy, body composition, sampling conditions or acute illness materially alter interpretation.
• Use serial calculations only when methods and units remain comparable.

Limitations

• Two points cannot confirm a truly log-linear phase.
• Assay error and timing error can markedly affect k.
• The model is unreliable with multi-compartment or changing clearance.

Clinical Pearls

• Use exact sample collection times rather than scheduled times.
• The earlier concentration must exceed the later concentration.
• Plot additional levels when the model is uncertain.

Common Mistakes

• Using a peak obtained before distribution is complete.
• Using times measured from different reference points.
• Ignoring an intervening dose.

Evidence Base

The implementation uses the published or established relationship: k = ln(C1 ÷ C2) ÷ (t2 − t1). The cited source should be reviewed alongside current specialty guidance before clinical deployment.

References

1. Sawchuk RJ, Zaske DE. Pharmacokinetics of dosing regimens which utilize multiple intravenous infusions. J Pharmacokinet Biopharm. 1976. — Primary publication, authoritative guideline or recognised clinical source supporting the calculation.