What is Identifiability?

Developing models for pharmacokinetic or pharmacodynamic data requires creativity, patience, and hard work. Sometimes that creativity ends up violating mathematical principles which can lead to poorly fitting models and frustration for a modeler. One common area where mistakes are made is related to the analysis of parent and metabolite data simultaneously. Because data are available … Continued

What is the -2LL or the Log-likelihood Ratio?

If you have ever read the literature on pharmacokinetic modeling and simulation, you are likely to have run across the phrase “-2LL” or “log-likelihood ratio”. These are statistical terms that are used when comparing two possible models. In this post, I hope to explain with the log-likelihood ratio is, how to use it, and what … Continued

Using Sampling “Windows” for PK Blood Samples

One of the most common questions posed by clinical operations experts when including pharmacokinetic sampling in a clinical trial is the following: “What is the time window we should allow for each blood sample?” My answer is always the same: “Don’t include any window.” I am almost always met with a confused look. The confusion is … Continued

What is “Adjusted” r-squared?

Linear regression is a common tool that the pharmacokineticist uses to calculate elimination rate constants. Standard linear regression provides estimates for the slope, intercept, and r2, a statistic that helps define goodness of fit. Statistical texts define r2 as the coefficient of determination and it is calculated using the following equation: where SS = the sum of … Continued

Demystifying CDISC, SDTM, and ADaM

Team discussions regarding CDISC often bring in the mists of darkness, which obscure the landscape and prevent us from moving in a clear direction. Then if we weren’t confused enough, the discussion moves to SDTM, ADaM, and clinical databases, and we feel like we are spinning out of control. The land of clinical study data can … Continued

Understanding Accelerator Mass Spectrometry

Accelerator mass spectrometry (AMS) is an analytical method used to detect the amount of radioactive carbon in a biological sample. It is an extremely sensitive methodology that can be used in early clinical research when conventional radiometric detection methods such as liquid scintillation counting are not possible. AMS is a powerful technique; however, it is … Continued

Why Do We Customize Aminoglycoside Dosing?

Aminoglycosides are a class of molecules that are used for the treatment of serious gram-negative systemic infections. Some common aminoglycosides are tobramycin, gentamicin, amikacin, and neomycin. Aminoglycosides have bactericidal activity against most gram-negative bacteria including Acinetobacter, Citrobacter, Enterobacter, E. Coli, Klebsiella, Proteus, Providencia, Pseudomanas, Salmonella, Serratia and Shigella. Aminoglycosides are also active against most strains of Staphylococcus aureus and S. epidermidis. Most strains … Continued

What is a Loading Dose?

Drug therapy in chronic disease situations requires systemic drug levels to reach target steady-state levels for maximum safety and efficacy. The time it takes for a drug to reach steady-state is a function of the elimination half-life of the drug. The following table illustrates how long it will take to achieve steady-state relative to the … Continued

The Superposition Principle

The superposition principle has nothing to do with a super-hero; however, you might be perceived as a hero if you can explain the principle to others. The superposition principle is a mathematical concept that helps us analyze concentration-time data. While it may seem complicated, it is actually nothing more than addition! The superposition principle states … Continued

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