Before Analysis: Sources of Variation

#Variation begins before collection

A test result reflects more than the condition being investigated. Food, exercise, hydration, posture and time of day can affect some measurements. Medicines and supplements may change the substance being measured or interfere with the measurement method. These effects differ between tests and are not always clinically important.

Preparation requirements are therefore specific to the test rather than universal. Some measurements require controlled timing or fasting, while others do not. In clinical practice, relevant preparation details and exposures help determine whether a result can be compared fairly with a reference interval or a previous measurement.

#Collection can alter the sample

How a sample is collected can introduce further variation. For example, damage to blood cells during collection or handling can release their contents into the sample. This is called haemolysis, and it can affect certain measurements without indicating that the same process is happening inside the body.

An unsuitable container, too little material or contamination can also affect results. A sample collected near an intravenous infusion may be diluted or contain substances from the infusion. For swabs and tissue, the collection site and the material actually obtained influence what the laboratory can detect.

#Handling affects stability

Changes can continue after collection. Delays, temperature, light exposure and storage conditions may alter cells, chemicals or microorganisms. Some samples need prompt processing; others remain suitable for longer. There is no single handling rule that protects every sample type and every measurement equally well.

Laboratories use acceptance checks and handling procedures to reduce these problems. When a result appears unexpected, the collection and transport history may help explain it. However, an unusual result should not automatically be dismissed as a sample problem: biological change, measurement limitations and other clinical explanations also need consideration.

#Common misunderstandings

A laboratory result is not shaped only by what happens inside the analyser. The sample can change before measurement, even when the testing equipment works correctly. This does not mean that every difference between results reflects an error: normal biological changes and measurement variability also contribute.

Fasting is another frequent source of confusion. It is not required for every test, and longer fasting is not automatically better. Preparation requirements depend on the measurement being made. Medicines, supplements, recent exercise and collection timing may matter, but their effects are not identical across tests.

A sample that looks normal is not necessarily unchanged. Some changes are invisible, while visible changes do not affect every measurement equally. Likewise, an unexpected result does not prove that collection or transport caused a problem. Clinicians may consider the preparation, collection conditions, laboratory comments and clinical context together before deciding whether another sample would help.

#Questions worth asking a clinician

• For this particular test, what should I do about fasting, medicines, exercise and timing before the sample is collected?
• Could the collection technique, container or contamination change this sample enough to affect the result?
• How soon must this sample be processed, and what temperature and storage conditions are needed to keep it stable?
• Were any collection problems or delays in transport, processing or storage recorded for this sample?
• If the result is unexpected, how will you distinguish biological variation from collection or handling problems and measurement error?