Blood sample analysis - Serum or plasma this is the difference

When you give a blood sample, you may think most about what is to be analyzed: blood lipids, blood sugar, iron or hormones. But in laboratory medicine, how the sample is taken and handled is also crucial. A common question is why some analyses are done on serum and others on plasma. For you as a patient, the words may sound almost identical, but for the laboratory, the choice can affect both the measurement value, the interpretation and how quickly an answer can be given. Understanding the difference between analyzing a blood sample – serum or plasma makes it easier to understand why different test tubes are used and why the same substance sometimes cannot be directly compared between different sample types. Both international laboratory guidelines and medical literature emphasize that sample type and preanalytical factors – that is, what happens before the actual analysis – are of great importance for the quality of blood test results.

What are serum and plasma in a blood sample?

Blood consists of blood cells and a liquid part. The liquid part contains, among other things, water, salts, proteins, nutrients, hormones and various waste products.

Plasma is the liquid obtained when blood is collected in a test tube that contains a substance that prevents the blood from coagulating, i.e. forming clots. The blood cells can then be centrifuged away, leaving plasma. Plasma still contains coagulation factors, including fibrinogen.

Serum is the liquid that remains after the blood has first been allowed to coagulate in the test tube. When the clot is then removed by centrifugation, the remaining liquid becomes serum. Since the coagulation process has already taken place, serum does not contain fibrinogen, and several coagulation factors are used up or changed.

This means in practice:

• Plasma = the fluid of the blood before coagulation

• Serum = the fluid of the blood after coagulation

The difference may seem small, but it is medically important. When the blood is allowed to clot, the contents of the sample change. Some substances are released from platelets and cells, others are broken down if the sample is left for too long, and some analyses require that the coagulation system is still intact.

Analysis of blood samples – serum or plasma: why does it matter?

In modern laboratory medicine, the type of sample is part of the quality of the test itself. A laboratory value is not only a measure of what is in the body, but also a result of which material is being analyzed.

There are several reasons why serum and plasma do not always give exactly the same results:

• Coagulation changes the content of the sample
  When blood clots, platelets and coagulation proteins are activated. This can affect the concentration of certain analytes, i.e. the substance being measured.

• Anticoagulants in the test tube can affect the analysis
  Plasma is taken in tubes that contain additives, such as heparin, EDTA or citrate. These are needed to prevent coagulation, but they can also make certain analyses unsuitable in that particular tube.

• The time to centrifugation matters
  Plasma can often be separated faster because you do not have to wait for the blood to clot. For analyses where rapid handling is important, it can provide better stability.

• Reference intervals and method are linked to sample type
  A normal value for serum is not always directly comparable to a normal value for plasma. The laboratory validates the analysis for a specific sample material.

For patients, this means an important thing: if you follow your blood values ​​over time, the samples should preferably be taken in the same way and analyzed with the same method, especially when it comes to markers where small differences can be significant.

Which analyses are done in serum and which are done in plasma?

There is no absolute rule that applies to all laboratories, but certain principles are well established.

Serum is often used for:

• many routine analyses in clinical chemistry

• liver samples

• blood fats

• some hormone samples

• antibody analyses and serology in many contexts

Plasma is often used for:

• coagulation tests such as PK(INR), APTT and fibrinogen

• analysis where rapid separation from blood cells is important

• some acute chemistry tests

• sampling where one wants to reduce changes that occur during coagulation

It is particularly important that coagulation analyses are performed on plasma, usually citrate plasma. This is because one wants to be able to measure the coagulation system before the blood has coagulated in the test tube. If serum were to be used, the coagulation process would have already occurred and the test would be misleading.

Another area where the type of sample plays a major role is glucose. Blood cells continue to consume glucose after sampling if the sample is not handled quickly and correctly. Therefore, guidelines emphasize that plasma is often preferable in the diagnosis of diabetes, since the sample can be centrifuged without first having to wait for coagulation. Delayed handling can lower the glucose value and give a falsely low level.

Potassium is also a good example. Serum values ​​can be slightly higher than plasma values ​​because platelets can release potassium during coagulation. In people with very high platelet or white blood cell counts, this can sometimes lead to an apparently elevated serum potassium, known as pseudohyperkalemia.

Serum and plasma - how the choice of test tube affects your test results

When a blood sample is taken at a health center, it may seem simple, but behind the scenes, a meticulous process is underway where every detail counts. The reason why the sampler often fills several different tubes with colored caps is because different medical analyses require completely different conditions. Depending on what is to be examined, the laboratory needs either serum or plasma, which are the two different forms of the liquid part of the blood. Using the right test tube is absolutely crucial for the analysis to be carried out at all and for the final result to be reliable.

Although everything that ends up in the tubes is blood, the additives and color codes of the tubes determine what happens to the sample after it is taken. Some tubes contain substances that cause the blood to coagulate in order to extract serum, while others contain additives such as EDTA or citrate that prevent coagulation in order to provide plasma instead. There is no general answer as to whether serum or plasma is best, but the choice is entirely determined by the specific analysis to be performed and what the method is validated for. For example, plasma is the obvious choice for coagulation studies, while serum works excellently for a wide range of other routine tests. If the wrong sample type is accidentally used, the analysis cannot be performed at all, or the results will be misleading.

For some analyses, the difference between serum and plasma may be clinically insignificant, while for others it is very clear. Classic examples of substances that are strongly affected by the sample type are glucose, potassium and certain coagulation factors. Handling immediately after the prick, the so-called preanalytical phase, is therefore critical. If a blood sample is left standing for too long before it is centrifuged and separated, the living blood cells continue to affect the fluid around them. This can lead to glucose levels dropping, potassium and LD rising, or the cells bursting, which is called hemolysis and interferes with a variety of analyses. As a patient, however, you don't need to worry about these details. The practical responsibility for choosing the right tube, handling the blood on time, and sending it to the laboratory according to all the rules of the art rests entirely with the healthcare and sampling team.

That's why sample handling is just as important as the blood analysis itself

When talking about blood tests, many people think of the instrument that measures the value. But a large part of the quality is created even before the sample reaches the analysis machine. This is called the preanalytical phase and includes, among other things:

• patient preparation

• correct test tube

• correct sample collection technique

• time to centrifugation

• transport and temperature

• separation of serum or plasma from blood cells

A checkup is only as valuable as the quality of the sample collection and interpretation. A correctly taken and correctly handled blood sample increases the chance of detecting abnormalities in time, monitoring changes over time and making the right decisions about lifestyle, follow-up or further care.

In practice, this means that two people with similar health status can get different test results if the sample collection is not standardized. Therefore, reputable laboratories work according to clear procedures to reduce sources of error and improve patient safety. This is especially true during preventive health checks, where small deviations are sometimes the very thing that provides an early signal of risk for, for example, diabetes, blood lipid disorders or other metabolic effects.

This is also why blood tests should be interpreted in context. A single value does not always tell the whole truth. You need to know:

• which analysis was taken

• whether the sample was serum or plasma

• whether the reference interval is adapted to the method

• whether the sample was taken fasting or non-fasting

• whether there are factors that may have affected the result

For those of you who want to understand your health better, it may be wise to ask not only “Is my value normal?” but also “How was the sample taken, and how should the result be interpreted?”

Understanding the difference between serum and plasma is fundamentally about understanding why laboratory medicine is so meticulous. It is not about details for the sake of details, but about each step contributing to the safest and most useful answer possible.