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"The right test, on the right specimen, collected at the right time, is essential for reliable diagnosis" Dr Prashant Goyal

Inter-Laboratory Variability: Why the Same Test May Produce Different Results

Have you ever undergone the same blood test at two different laboratories and received slightly different results? It can be confusing and sometimes worrying. The immediate thought is often, “Which report is correct?”

The answer is that a difference does not necessarily mean that one laboratory is wrong. Laboratory results are measurements, and like all measurements, they have some degree of analytical variation and biological variation.

Pre-analytical variation is one of the most important and frequently overlooked causes of variation. The following factors can influence laboratory results:

  • Fasting or non-fasting status
  • Time of sample collection
  • Exercise before collection
  • Posture
  • Stress
  • Dehydration
  • Prolonged tourniquet application
  • Difficulty during blood collection
  • Hemolysis
  • Lipemia
  • Icterus
  • Delay in transportation
  • Improper temperature during transportation
  • Incorrect tube
  • Inadequate mixing
  • Delayed separation of serum/plasma

 There can also be analytical variation. Different laboratories may use different instruments, reagents, calibrators, or testing methods. As a result, the same sample may produce different numerical values on different analytical platforms. This is particularly relevant for some hormones, immunoassays, antibodies and specialised tests. Therefore, when monitoring a patient over time, it is often preferable to perform serial testing using the same laboratory and, where possible, the same analytical method.

Another important factor is biological variation. Our bodies are not chemically constant. Glucose, hormones, triglycerides and many other parameters naturally fluctuate during the day or from one day to another. Therefore, even within the same laboratory, values may differ when the test is repeated.

For example:

  • Glucose changes with meals, exercise and stress.
  • Cortisol varies significantly according to time of day.
  • Triglycerides can change with food intake.
  • White blood cell counts can fluctuate with infection, stress and other physiological factors.
  • Hormonal (TSH, Prolactin, LH, FSH etc) levels may change according to physiological cycles.
  • Some markers vary with hydration status.

Reference ranges can also vary between laboratories. A result that is considered normal in one laboratory may appear slightly outside the reference interval in another because the reference population or analytical method is different.

However, a large or unexpected difference should not be ignored. If a result does not fit the patient's symptoms or previous reports, the laboratory and clinician may need to investigate possible pre-analytical problems, analytical interference, method differences or a genuine change in the patient's condition.

Modern laboratories use internal quality control, calibration, method verification and external quality assessment to minimize such variability. Yet, zero variation between laboratories is neither realistic nor necessary.

So, when two laboratories give different results, don't immediately ask, “Which laboratory is wrong?”

Instead, ask: “Is this difference expected, or is it clinically meaningful?”

Different does not always mean wrong. What matters is whether the result is reliable, comparable and clinically useful.


- Dr Prashant Goyal 


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