Is creatinine worth drawing at baseline before starting a GLP-1 receptor agonist?

What I am working with: creatinine · a GLP-1 receptor agonist.

I am trying to build something sustainable rather than something thorough that I will abandon.

I have already decided the broad direction; this is about the specifics.

What should I decide now, and what should I defer?

Answering this needs to distinguish screening from monitoring. A screening panel looks for the unexpected; a monitoring panel tracks something you already have a reason to watch.

Timing matters per analyte: cortisol and testosterone are diurnal, triglycerides are postprandial, and creatinine responds to hydration and to recent training. Fixing the conditions removes most of the noise.

Repeat before you react. A single abnormal value has a substantial probability of being within the combined biological and analytical variation of a normal one.

Pre-analytical factors — posture, tourniquet time, fasting, sample handling — are the largest source of error in routine biochemistry, well ahead of the analysis itself.

Same laboratory, same time, same fasting state, or the comparison is not a comparison.

Before reacting to any single value, check whether it is outside the interval by an amount larger than the assay's own variation.

A twenty-analyte panel run on a healthy person will produce, on average, one out-of-range result purely from how reference intervals are constructed. That is arithmetic rather than pathology.

The underlying point is that delta checks — comparing against your own previous value — are far more sensitive than comparing against a population interval, which is the argument for keeping a series rather than a snapshot.

One out-of-range value on a twenty-analyte panel is expected. Two on a repeat is a finding.

edited 28 Jul 2025 by coldpack_88 — added the method parameters

Standardise the conditions — same time of day, same fasting state, same laboratory — or you are measuring the conditions rather than yourself.

Keep the reports rather than the numbers. Units, reference intervals and methods all vary, and a bare number two years later is not comparable to anything.

External quality assurance schemes document between-laboratory differences on common analytes that routinely exceed the size of clinically interesting changes.

Baseline first, then a repeat under identical conditions. Everything else is secondary.

The honest position is that most people order too many analytes and too few time points, when the reverse would be more informative.

A sensible core for this population is a full blood count, renal function with electrolytes, liver enzymes with bilirubin, a fasting lipid panel with apolipoprotein B, HbA1c and thyroid-stimulating hormone.

Reference intervals are conventionally the central ninety-five per cent of a reference population, which is the direct cause of the one-in-twenty out-of-range rate on a healthy panel.

Research-use compounds are not approved for human use, and no panel makes that safer.

Decide the action for each result before you order the test.