What interval makes sense for repeating creatinine on a GLP-1 receptor agonist?

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

I would like to set this up properly once, rather than adjust it repeatedly.

My budget is real but not tight, and my tolerance for uncertainty is low.

What would you do, and what would make you change course?

Put another way, this is answerable, and the answer is mostly about which tests rather than how many.

Same laboratory, same method, same time of day, same fasting state. Between-laboratory differences on several common analytes are larger than the changes people are trying to detect.

Haemolysis in the sample raises potassium and several enzymes spuriously. If a result is bizarre, ask whether the sample was flagged before building a theory on it.

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

Nothing here is medical advice. If something is out of range and you do not know why, that is a consultation rather than a research project.

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

Start with a baseline. A result taken before anything started converts most later ambiguity into a simple comparison, and it cannot be obtained retrospectively.

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.

It helps to be literal here: 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.

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

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.

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

Keep the full report, not the number. You will need the units and the interval later.

edited 29 Oct 2024 by Dr_Nadia_Farsi — corrected a unit error in the worked example

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

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.

The caveat is that a panel is not a diagnosis and interpreting one is a clinician's job, particularly when several values move together.

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

edited 14 Nov 2024 by Dr_Sara_Kuusela — expanded the table to cover the lower concentration