Before Analysis: Sources of Variation

#The process starts before measurement

The preanalytical part of laboratory testing includes what happens before the measurement: preparation, identification, collection, handling and transport. WHO's laboratory quality handbook treats these as planned processes with information passed between the requester, collector and laboratory.

A technically precise measurement cannot make a wrongly identified or unsuitable sample representative of the intended person and question. The request and sample record need the right context, including the requested tests and collection details. This explains why laboratory quality work extends beyond calibrating an instrument.

Evidence: WHO: sample collection, preservation and transport / Ozarda: reference interval methods and limits

#Preparation depends on the particular test

Eating, medicines or supplements, strenuous exercise and collection timing can affect some results. MedlinePlus explains that many tests need no special preparation, while others have specific requirements. WHO notes that timing may matter for tests such as drug levels and hormones. There is no single preparation rule for all laboratory testing.

Preparation belongs to the instructions for the requested test and the clinical team. MedlinePlus specifically warns against stopping medicines unless the provider instructs it. This page supplies no fasting duration or medication change. If preparation differed from the instructions, that information needs to reach the clinical or laboratory team rather than be hidden from interpretation.

Evidence: MedlinePlus: preparing for a lab test / WHO: sample collection, preservation and transport

#Collection and transport must fit the specimen

The required specimen may be serum, plasma, whole blood or another material. WHO describes how the container, volume, additives, labelling and special handling depend on the requested test. Some samples require protection from light, controlled temperature or prompt delivery. These are test-specific requirements, not universal instructions for a reader to apply alone.

Transport conditions need to preserve sample integrity as well as keep handlers safe. Collection and receipt times help laboratories assess whether their conditions were met. A result can be delayed or require recollection when the sample is not suitable. An online reference range cannot repair a collection or transport problem.

Evidence: WHO: sample collection, preservation and transport

#Sample-quality comments change what can be concluded

Hemolysis is the breakdown of red blood cells with release of their contents. The EFLM preanalytical working group explains how this can interfere with clinical chemistry measurements. The effect depends on the measured substance and method; there is no general rule that every affected value moves in the same direction.

The EFLM paper discusses test-specific assessment and comments about possible interference. Its scope is clinical chemistry in adults and older children, not every laboratory test or age group. A sample-quality flag or an unavailable result should therefore be read with the laboratory's explanation and clinical context. This page does not apply the paper's suppression thresholds, correct a value or decide whether someone needs another sample.

Evidence: EFLM WG-PRE: hemolysis and test interference / WHO: sample collection, preservation and transport

Source note

The sections above were checked against the linked sources. No clinical review has been performed. This is general research education, not a clinical guideline.