Why did my eGFR move after six weeks on oral semaglutide?

The particulars: eGFR · six weeks · oral semaglutide.

Before I write this off, I want to check whether it is a known failure mode.

I have the lot number, the certificate and the date, and I am happy to compare them against anything.

What would you check first, and what would you conclude from each outcome?

6 weeks is 42 days, and the first question about any marker is whether 42 days is long enough for it to have finished moving. An eGFR calculated from creatinine is partly a muscle-mass measurement wearing a kidney-function label, and both that and the hydration behind it move early in a deficit. Against 42 days that puts the marker well inside its own settling time, so the value is reporting a new steady state rather than a transient. The second question is the denominator. Weight loss moves plasma volume, muscle mass and intake at once, and several of the markers on a routine panel are ratios with one of those three underneath them. Repeat before interpreting. A single value 42 days in, with no baseline drawn under the same conditions, is a number rather than a change — and nothing here is medical advice.

Answer first: decide what you would do differently for each possible result before you order the panel. Anything that fails that test is a number you will worry about and not act on.

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.

Relative to absolute, worked

The last two rows describe the same finding. Only one of them is used in headlines.

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.

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.

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.

The short version: a small, well-chosen panel with a baseline beats a large one without.

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.

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.

Biological variation data are published per analyte and are the basis for the reference change value — the difference between two results that is larger than noise.

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

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

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.

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.

Ordering tests you will not act on generates anxiety and incidental findings, both of which have costs.

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

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

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.

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 5 Jan 2025 by Dr_Rosalind_Achebe — added a caveat about sampling

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

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.

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

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