Why Do Laboratory Test Results Differ Between Laboratories?
Some people are surprised to receive different results when the same laboratory test is performed at two different laboratories. This often raises questions such as:
Did one of the laboratories make a mistake?
Was the first result inaccurate?
Should the test be repeated?
In reality, a difference in laboratory results does not necessarily indicate an error. In many cases, the variation is minor, scientifically expected, and may result from factors related to the patient, sample collection, laboratory equipment, reagents, or even the reporting process. For this reason, modern laboratories follow strict quality assurance procedures throughout every stage of the testing process—from patient preparation to the release of the final report.
Is It Normal for Laboratory Results to Differ?
The short answer is yes—but only within scientifically acceptable limits.
Laboratory tests are highly accurate; however, they do not represent fixed values. Many biological markers naturally fluctuate throughout the day and can be influenced by food intake, physical activity, sleep, stress, medications, and even body position during blood collection. As a result, slight differences in test results may occur over time, even when the test is performed in the same laboratory.
For example:
Blood glucose levels vary between the fasting and post-meal states.
Certain hormones fluctuate according to the time of day or the menstrual cycle.
White blood cell counts may temporarily increase after intense physical activity or emotional stress.
Therefore, physicians do not interpret laboratory values in isolation. Instead, they evaluate the results alongside the patient’s clinical condition, symptoms, medical history, and other diagnostic findings.
Important Fact
A small difference between two laboratory results does not automatically mean that one of them is incorrect. The first step is to determine whether the difference falls within scientifically acceptable limits and whether it can be explained by known physiological or laboratory-related factors
How Does a Sample Travel Through the Laboratory?
Before your laboratory result appears on your phone or in your report, your sample undergoes a series of carefully controlled steps known as the Total Testing Process (TTP).
This process consists of three major phases:
🔹 Pre-Analytical Phase
This phase begins when the test is requested and includes patient preparation, sample collection, transportation, and receipt of the sample in the laboratory.
🔹 Analytical Phase
During this phase, the sample is analyzed using specialized laboratory instruments and reagents while both internal and external quality control procedures are applied.
🔹 Post-Analytical Phase
This final phase includes reviewing the results, verifying their accuracy, approving the report, and delivering it to the physician or patient.
Studies have shown that a significant proportion of factors affecting laboratory results occur before the analytical phase begins—during patient preparation, sample collection, handling, or transportation rather than during the analysis itself. This is why accredited laboratories place great emphasis on this stage to ensure the highest possible level of accuracy.
First: The Pre-Analytical Phase
The pre-analytical phase begins before the sample reaches the analyzer and includes every step that takes place prior to laboratory testing, such as patient preparation, sample collection, storage, transportation, and reception.
Although many people believe that laboratory accuracy depends solely on the analyzer, research has shown that most variables affecting laboratory results originate during the pre-analytical phase. Consequently, modern laboratories implement strict procedures to ensure sample integrity before analysis.
1. Failure to Follow Preparation Instructions
Preparation requirements vary depending on the type of test. Failure to follow these instructions may lead to inaccurate results.
For example:
Eating before certain tests may affect blood glucose and triglyceride levels.
Certain medications can alter the results of various laboratory tests.
Intense physical exercise before blood collection may temporarily increase biomarkers such as Creatine Kinase (CK) and lactate, and may also influence some components of the complete blood count (CBC).
For this reason, patients should always follow the preparation instructions provided by their physician or laboratory.
2. Timing of Sample Collection
Not all laboratory tests remain constant throughout the day. Some biological substances naturally fluctuate according to the body’s circadian rhythm.
Examples include:
Cortisol, which reaches its highest concentration in the morning and gradually declines throughout the day.
Certain hormones that should be measured at specific times to ensure accurate clinical interpretation.
Therefore, performing the same test at different times of the day may produce different results without indicating any underlying health problem.
3. Body Position During Blood Collection
This may come as a surprise, but body position before blood collection can influence certain laboratory results.
Remaining seated or standing for an extended period before blood sampling may cause slight changes in the concentration of some blood components due to shifts in body fluids between blood vessels and surrounding tissues.
For this reason, most laboratories ask patients to remain seated and relaxed for a few minutes before blood collection.
4. Sample Collection Technique
The accuracy of laboratory results also depends on proper blood collection.
For example, improper collection techniques may cause hemolysis, the breakdown of red blood cells within the sample. Hemolysis can interfere with several laboratory tests, including:
Potassium
Lactate Dehydrogenase (LDH)
Aspartate Aminotransferase (AST)
In such cases, the laboratory may request a repeat blood sample to obtain a more reliable result.
5. Type of Collection Tube
Not all blood samples are collected in the same type of tube.
Each laboratory test requires a specific collection tube containing additives designed to preserve the sample or prevent clotting.
Using an inappropriate collection tube may lead to inaccurate results. Therefore, laboratories follow standardized protocols to ensure the correct tube is used for every test.
6. Sample Storage and Transportation
The pre-analytical phase continues even after the sample has been collected.
Some specimens must be analyzed within a specific time frame, while others require transportation and storage under carefully controlled temperatures.
Any delay or exposure to inappropriate temperatures may alter certain sample components and consequently affect the laboratory results.
For this reason, laboratories use dedicated transportation systems and continuously monitor storage conditions until the sample reaches the analytical section.
Can Laboratories Minimize These Factors?
Yes.
Modern laboratories implement comprehensive quality procedures to minimize the impact of these variables, including:
Verifying the patient’s identity before sample collection.
Confirming compliance with preparation instructions.
Using the appropriate collection tube for each test.
Training phlebotomy staff according to recognized standards.
Transporting samples under controlled time and temperature conditions.
Rejecting samples that do not meet quality requirements when necessary.
These measures help ensure that laboratory results accurately reflect the patient’s true clinical condition.
Important Note
If a laboratory asks you to provide a new blood sample, this does not necessarily mean that there is a problem with the analyzer or that the laboratory made an error. In many cases, it is simply a quality assurance measure to ensure that the sample is suitable for analysis and that the final result is accurate, reliable, and clinically meaningful.
Second: The Analytical Phase
Once the sample has been received and verified to meet quality requirements, it enters the Analytical Phase. During this stage, the sample is examined using advanced laboratory instruments and technologies to produce the required test results.
This is the most closely monitored phase of the laboratory testing process. Modern laboratories follow strict quality assurance procedures to ensure that every result is accurate, reliable, and clinically meaningful before it is released to the physician or patient.
Do All Laboratories Use the Same Instruments?
The answer is no.
Many internationally recognized companies manufacture laboratory analyzers, and each uses its own measurement technologies and methodologies. Examples include:
Roche
Abbott
Siemens Healthineers
Beckman Coulter
Sysmex
Mindray
Bio-Rad
Although these manufacturers use different technologies, their instruments must comply with rigorous quality standards and demonstrate accuracy and reliability before being used in clinical laboratories.
Therefore, using different analyzers does not necessarily result in differences in the quality of laboratory results. In some cases, minor variations may occur; however, these generally remain within scientifically acceptable limits.
What Are Reagents?
Reagents are chemical or biological substances used by laboratory analyzers to measure specific components in a patient’s sample, such as:
Glucose
Cholesterol
Hormones
Enzymes
Tumor markers
These reagents are manufactured with high precision and must be stored under specific temperature conditions while strictly following expiration dates and the manufacturer’s recommendations to ensure consistent and reliable performance.
What Is Calibration?
Even the most advanced laboratory analyzers require routine calibration.
Calibration is the process of adjusting an instrument using reference materials with known values to ensure that the measurements produced by the analyzer accurately reflect the true values.
A simple analogy is an electronic weighing scale. If it is not calibrated correctly, it may produce inaccurate measurements even though it appears to function normally.
For this reason, laboratory analyzers are calibrated regularly, whenever reagents are changed, or after maintenance has been performed, following both manufacturer recommendations and laboratory quality procedures.
What Is Internal Quality Control (IQC)?
Before patient samples are analyzed, laboratories do not begin testing immediately.
Instead, they first analyze quality control materials containing known target values.
The analyzer compares the measured results with the expected reference values.
If the results fall within the acceptable range, patient testing proceeds.
If the results fall outside the acceptable limits, testing is suspended until the cause has been identified and corrected.
This process helps detect potential analytical problems before they can affect patient results.
What Is External Quality Assessment (EQA)?
In addition to internal quality control, high-quality laboratories participate in External Quality Assessment (EQA) or Proficiency Testing (PT) programs.
In these programs, laboratories receive unknown samples from independent organizations.
The samples are analyzed exactly as patient specimens, and the laboratory’s results are then compared with those obtained by hundreds or even thousands of laboratories using similar methodologies worldwide.
These programs provide an objective assessment of laboratory performance and help ensure compliance with international quality standards.
Do Different Instruments Produce Different Results?
Not necessarily.
Different laboratories may use analyzers or reagents from different manufacturers while still producing equally accurate and clinically reliable results.
For some laboratory tests, slight variations may occur because of differences in analytical methods or reagent formulations. However, these differences usually remain within scientifically acceptable limits and rarely affect medical decision-making when interpreted appropriately.
For this reason, physicians often recommend performing serial follow-up tests, particularly for chronic conditions such as diabetes, thyroid disorders, or tumor marker monitoring, in the same laboratory whenever possible. This helps minimize minor analytical variations when comparing results over time.
Are Laboratory Results Reviewed Before Release?
Yes.
In laboratories that operate under effective quality management systems, results are not automatically released once the analyzer has completed testing.
Instead, they undergo a professional review that includes:
Verifying patient identification and sample information.
Reviewing abnormal or critical values.
Comparing current results with previous laboratory findings when appropriate.
Confirming that there are no indicators of sample-related or analytical problems.
In some situations, the laboratory may repeat the analysis before approving the final report to ensure result accuracy.
Important Note
The accuracy of laboratory testing does not depend solely on the analyzer. It relies on an integrated quality system that includes high-quality instruments, validated reagents, regular calibration, internal and external quality control, qualified laboratory professionals, and strict adherence to standardized operating procedures.
Laboratories that consistently implement these quality measures are better equipped to provide results that physicians can trust when making clinical decisions.
Third: The Post-Analytical Phase
Many people assume that the laboratory testing process ends once the analyzer has completed testing. In reality, another equally important stage begins—the Post-Analytical Phase.
This phase involves reviewing the laboratory results, verifying their accuracy, approving the final report, and delivering it to the physician or patient.
Laboratories operating under comprehensive quality management systems do not release results automatically. Instead, they follow a structured review process designed to ensure that every report is accurate, reliable, and suitable for clinical decision-making.
Reviewing Results Before Release
Before the final report is issued, laboratory professionals review several important aspects, including:
Confirming that the patient information matches the submitted sample.
Reviewing abnormal or critical laboratory values.
Comparing current findings with previous laboratory results when appropriate.
Ensuring there are no indications of sample-related or analytical issues.
In some cases, the laboratory may repeat the analysis—or even request a new specimen—if the quality of the original sample could compromise the accuracy of the results.
Should You Always Use the Same Laboratory?
Not necessarily.
If laboratories follow recognized quality standards, their results should generally be comparable for most routine laboratory tests.
However, physicians often recommend using the same laboratory for follow-up testing in chronic conditions such as diabetes, thyroid disorders, or tumor marker monitoring whenever possible.
This recommendation is not because other laboratories are less accurate, but because using the same analyzer, reagents, and reference intervals improves the consistency of long-term result comparisons.
When Are Differences in Results Considered Normal?
Minor differences in laboratory results may be expected in situations such as:
Testing on different days or at different times.
Variations in fasting status or food intake.
Recent physical exercise.
The use of medications that influence laboratory values.
Natural biological variation within the human body.
Minor methodological differences between laboratory instruments that remain within acceptable scientific limits.
Therefore, a small difference between two laboratory results does not necessarily indicate that one of them is incorrect.
When Should Differences in Results Be Discussed with a Physician?
You should consult your physician if:
The difference between results is unexpectedly large.
The laboratory findings do not match your symptoms or clinical condition.
Critical or significantly abnormal values appear unexpectedly.
Your physician recommends repeating the test or performing additional investigations.
Laboratory results should always be interpreted by a qualified healthcare professional within the context of the patient’s medical history, physical examination, and overall clinical picture—not based solely on numerical values.
How to Choose a Reliable Laboratory
When selecting a medical laboratory, consider the following factors:
Commitment to comprehensive quality management, including internal and external quality control.
Use of modern laboratory analyzers and advanced diagnostic technologies.
A qualified and well-trained laboratory team.
Strict adherence to standardized procedures for sample collection and analysis.
Clear, accurate, and reliable laboratory reports.
Prompt handling and reporting of critical laboratory results when necessary.
Together, these elements enhance the reliability of laboratory testing and support physicians in making accurate clinical decisions.
