Fasting before blood tests matters for a handful of analytes, not the panel

Design

Retrospective single-centre cross-sectional study with within-person analyses

Population

101,148 adult outpatients; 9,755,547 results across 372 tests

Primary outcome

Differences by fasting duration beyond analytical performance limits

Effect

10 analytes confirmed; under 8 h: glucose +10.8%, triglycerides +20.1%, lipase +36.4%

A retrospective study at one Korean hospital analysed nearly 9.8 million results from 101,148 adult outpatients across 372 tests, grouped by time since the last meal: under 8 hours, 8 to 12, and over 12. Differences were judged against laboratory standards for clinically meaningful change, and within-person comparisons in repeatedly tested patients were used to strip out confounding.

Only 10 tests showed meaningful differences within the same person. The main ones were higher after less than 8 hours of fasting: glucose by 10.8%, triglycerides 20.1%, GGT 14.8% and lipase 36.4%. Nothing differed meaningfully between 8 to 12 hours and more than 12 hours.

This supports test-specific fasting instructions rather than a blanket overnight fast, which inconveniences patients, skews morning clinics and is a hypoglycaemia risk for people on insulin or sulfonylureas.

• Ask for fasting only when ordering fasting glucose, triglycerides, GGT or lipase where precision matters.
• Most other routine tests can be drawn without fasting.
• Fasting beyond 12 hours adds nothing and should not be requested.
• Note the time of the last meal on the request when a non-fasting sample is sent for glucose or lipids.

Why it matters

Blanket fasting instructions cost patients a morning and put those on glucose-lowering drugs at risk for little analytical gain.

Don't overread it

A single-centre, observational study; whether it changes any clinical decision was not tested.

The statistics, in plain English

With millions of results, almost any difference reaches statistical significance, so the authors compared differences with established limits of clinically meaningful change. Twenty-seven tests crossed that line between groups, but only ten within the same person, showing how much of the apparent effect came from who fasted, not the fasting.

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