What are the differences between serum and plasma? Why do we need to collect these samples for drugs, biologics, and other drug therapies? Let’s discuss how serum and plasma are used in animal studies to understand drug safety and efficacy. Get the answers you need at Anilocus.

At Anilocus, we receive many inquiries from Sponsors regarding serum and plasma and the differences between the two. This article will explore what we can learn from analytical testing about drug safety and efficacy and review the distinctions between serum and plasma. Moreover, we will discuss how plasma and serum are used in pharmacokinetics (PK) studies to better understand drug absorption, distribution, metabolism, and excretion.

Anilocus a premier preclinical in vivo CRO discusses the difference between plasma and serum. In this image of blood sample test tubes, the tubes with the light blue, cyan color blood collection tubes are for plasma and contain sodium citrate additive. The red-top blood collection tubes are for serum. They do not contain additives.
In this photograph, the light blue blood collection tubes are for plasma analysis and contain 3.2% sodium citrate additive while the red top tubes on the right are for serum analysis and do not contain any additives.

What is blood and how is it collected from animals for in vivo drug studies?

Blood is a vital component of all living organisms, and it plays an important role in providing essential nutrients and oxygen to all the cells in the body. It also helps to remove waste products from the body. Blood is composed mainly of red blood cells, white blood cells, platelets, and plasma.

In vivo drug studies involve the collection of blood from animals, usually rodents, to assess the concentration of a drug in the blood over time. The amount of blood collected from a single live rodent typically ranges from 0.1 to 0.5mL. Terminal blood collection allows us to collect over 1.0mL of whole blood depending on the size of the animal. We collect the sample using different blood collection tubes depending on the endpoint analysis.

How do I collect serum from an animal?

If an endpoint analysis calls for only serum, we collect this using a serum separator tube that has an added clotting factor. The serum is collected from an animal by first drawing blood into a tube containing a clotting activator or no anticoagulant. We then centrifuge the tube to separate the serum from the blood cells and other components.

What proteins are found in serum?

The proteins found in serum are albumin, globulins, fibrinogen, and antibodies. Albumin is the most abundant protein in serum, making up about 50-60% of the total protein content. Globulins make up the second most abundant protein and include immunoglobulins, complement proteins, and clotting factors. Fibrinogen is the third most abundant protein in serum and is involved in the clotting of blood. Antibodies are present in small amounts in serum and are involved in the immune response.

What is serum used for in animal studies?

The serum is used in animal studies to measure the levels of various proteins, hormones, enzymes, and other substances in the blood. This information can then be used to assess the health of the animal or to monitor the progress of a particular study. For example, we collect serum for Hormone Multiplex ELISA Panels or by HPLC. In these assays, we suggest measurements for gonadal hormone panels for Developmental and Reproductive Toxicology (DART) studies if there are drug-associated risk factors for fertility and pregnancy. We collect samples from males, females, pregnant dams, and/or neonates to measure a variety of hormones such as luteinizing hormone, estrogen/estradiol, progesterone, androgens, testosterone, anti-Müllerian hormone (AMH), etc.

How do I collect plasma from an animal?

Collecting plasma from an animal requires taking a blood sample from the animal. This is done by inserting a needle into a vein, artery, or cardiac puncture. Once the sample is taken and collected in tubes that are EDTA-treated (lavender tops), citrate-treated (light blue tops), or Heparinized tubes (green tops). The blood is centrifuged to separate the plasma from the red blood cells. This process takes several minutes and depends on the speed of the centrifuge. After the plasma is separated, it can be collected and stored at cold temperatures for use in experiments. We utilize plasma for Plasma Protein Binding mass spectroscopy assays required for pharmacokinetics.

What proteins are found in plasma?

Plasma contains proteins such as albumin, globulins, and fibrinogen. Albumin helps to maintain blood volume, regulates pH, and transports substances such as drugs and hormones. Globulins are involved in immune responses, blood clotting, lipid transport, and hormone transport. Fibrinogen is a key factor in the clotting of blood. Other proteins found in plasma include antibodies, antigens, enzymes, and complement proteins.

What is plasma used for in animal studies?

Plasma is used in animal studies to measure the levels of various cells and other substances. It can be used to assess the health of the animal or to monitor the progress of a particular study.

What are the benefits of allowing a blood sample adequate time for clotting before it is centrifuged?

Blood clotting time is important prior to centrifuging blood samples because it helps to ensure accurate results and proper separation of serum and plasma for analytical testing. If the sample is not clotted properly, the centrifugation process can cause the cells to rupture and release their contents, which will contaminate the sample and make it unsuitable for testing. Additionally, when clotting is not done properly, the sample may contain excessive procoagulant or anticoagulant proteins that will affect the accuracy of the results.

What is the difference between serum and plasma for testing purposes?

For testing purposes, the main difference between serum and plasma is that serum does not contain any blood cells or platelets, while plasma does. Therefore, serum can be used for more precise testing, as it can detect smaller quantities of proteins and other substances.

What are the advantages of using serum or plasma in animal studies?

The primary advantage of using serum or plasma in animal studies is that they provide an accurate and reliable measure of the levels of various substances in the blood. This information is used to determine: ADME (absorption, distribution, metabolism, and excretion) for pharmacokinetics. We utilize mass spectroscopy for tissue including serum and plasma.

What drug modalities require in vivo ADME analysis?

In pharmacokinetics, In vivo ADME analysis or absorption, distribution, metabolism, and excretion analysis is an important preclinical step of drug discovery and development. This analysis is needed to determine the safety and toxicity of the drug as well as its efficacy through dosage optimization in pharmacodynamics studies. The ADME analysis is necessary for different drug modalities such as small molecules, biologics, gene therapies, and natural products such as herbs and vitamins.

Small molecules are generally the most well-studied in this type of analysis, as they can be more easily tested in an animal or human system than biologics and natural products. For small molecules, in vivo ADME is needed to measure the pharmacokinetics of the drug which includes its absorption and dispersal throughout the body, the rate of metabolization, and how quickly it is excreted from the body.

Biologics are a relatively new category of drug that requires more complex testing due to their unique molecular structure. For biologics, this analysis is important to evaluate the ability of the body to absorb, use, and eliminate antibodies and proteins such as vaccines, monoclonal antibodies, and growth factors. Gene therapies also require in vivo ADME analysis to study the pharmacokinetics of nucleic acid-based drugs.

Natural products may be some of the most difficult to analyze due to their complexity and variability, but this analysis still yields valuable information for pharmacokinetic studies.

References

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  3. Yu Z, Kastenmüller G, He Y, et al. Differences between human plasma and serum metabolite profiles. PLoS One. 2011;6(7):e21230. doi:10.1371/journal.pone.0021230.
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  5. Rafidi H, Rajan S, Urban K, et al. Effect of molecular size on interstitial pharmacokinetics and tissue catabolism of antibodies. MAbs. 2022;14(1):2085535. doi:10.1080/19420862.2022.2085535.