Is cystatin C worth drawing at baseline before starting semaglutide?

What I am working with: cystatin C · semaglutide.

I am at the decision point and I would rather think it through than improvise.

I would rather spend money on measurement than on redundancy.

What does a sensible plan look like, and what are the decision points?

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

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.

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.

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

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.

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.

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.

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

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

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

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.

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

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

edited 5 Jul 2026 by v_ramaswamy — corrected a unit error in the worked example

The relevant statistical point is that a ninety-five per cent reference interval means one analyte in twenty will read out of range in a healthy person by construction.

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.

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

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