Serum vs. Plasma: What to Use for Your Research?

Serum vs. Plasma: What to use for your research?

Blood is routinely collected because circulating analytes provide a readout of the systemic health of the individual through a minimally invasive approach. The blood contains many metabolites, proteins, and cell types that can be measured to evaluate healthy versus disease conditions, environmental factors, lifestyle and nutritional effects or the activity of therapeutics and other interventions. Collection and storage of blood-derived samples must be performed using appropriate procedures for the downstream needs of the researcher, so proper planning and considerations in advance of study initiation are important. The most commonly collected blood-derived samples are serum or plasma which are devoid of any cells but have many different analytes that can inform on systemic health and provide diagnostic value. Separating out the cellular fraction is necessary due to the unique freezing requirements required to preserve cells and to eliminate interference from cellular factors that could be released through hemolysis during freeze/thaw cycles. 

The basics: Serum vs. Plasma  

Both plasma and serum represent the cell-free fraction of blood, separated from the cells by centrifugation, with slight differences due to the way they are processed. Serum is allowed to clot prior to centrifugation which removes fibrin clots, blood cells and related coagulation factors. During the process of coagulation for serum collection, platelets release proteins and metabolites into the serum and the enzyme thrombin cleaves fibrinogen to fibrin, an insoluble protein, that can be separated from the sample.(1) Plasma, on the other hand, is first treated with anticoagulants (i.e., EDTA, heparin, citrate) before the blood cells are removed by centrifugation, so that clotting factors remain deactivated in the liquid. In plasma, fibrinogen is kept in solution with anticoagulants. 

Using plasma or serum can affect the metabolome or proteome and affect study results.(1,2) Another consideration is that the blood collection tubes for serum can also have separator gels that create a barrier between the cells and the serum during centrifugation and this can also affect the metabolic profile produced following analysis.(2) While these gels may influence results slightly, they can also improve analyte stability separate fractions more clearly. If collecting samples from a biobank, ensuring all samples were collected using the same methods is critical for generating reliable data in subsequent downstream analyses.  

What are they used for?

Serum forms a larger percentage of the blood than plasma and depending on your research needs or focus, it may be preferred to plasma as it does not utilize any additive factors such as EDTA. While lipoprotein levels are often lower in plasma than serum, lipoprotein composition in plasma and serum is typically consistent within the same sample.(2) So overall, the biochemistry of each sample, regardless of collection method is expected to remain consistent, but it is important within a single study to utilize the same collection method to be able to make valid comparisons. The same is true if trying to compare results from multiple studies – confirming that the blood collection choice is consistent is critical to analyzing the data. Serum provides a higher sensitivity than plasma, so metabolite concentrations are generally higher. As such, serum can make it easier to identify more biomarkers than plasma in studies when comparing across various phenotypes or subpopulations. However, metabolite results from plasma samples tend to have better reproducibility from sample to sample.(1)  For some applications, diagnostic standards have been established using a particular collection approach, so not using the same sample type can lead to improper diagnoses. For example, matrix metalloproteinases, which are utilized as biomarkers of atherothrombotic risk and predictors of coronary cerebrovascular disease recurrence, should be analyzed from citrate plasma only for clinical and diagnostic purposes as other methods can interfere with activity and generate preanalytical biases.(3) 

Current Uses in Drug Discovery Research and Clinical Practice

Both serum and plasma are used often for medical research purposes to test for certain biomarkers or identify new ones related to a certain condition or disease. Preferences for sample type can vary across fields and downstream technologies being utilized for analysis. For example, for proteomic approaches, plasma is typically preferred. Appropriate sample handling for biomarker applications is critical as platelet activation and contamination can affect quantification of many molecules, particularly for cytokine profiles. 

One of the biggest concerns related to serum or plasma collection is being able to control for sample collection differences that can cause variability in results. For some applications, certain markers can be used as a proxy for measuring the quality of the plasma or serum sample. For example, platelet factor 4 (PF4) levels reflect activation and degranulation of platelets, and thus can be measured to represent a marker of platelet contamination from sample to sample.(4–6) Sample handling considerations, such as biospecimen preparation, temperature, delivery, freezing, storage and thawing can impact data variability. These can often be controlled in a prospective study with planning and quality practices, but collecting samples across various studies or retrospectively from biobanks may cause systematic variation. 

In addition to measuring metabolites, proteins, and lipids, cell-free circulating DNA can be a valuable biomarker for detecting genetic or epigenetic alterations related to cancer development and progression. As cell-free circulating DNA is released from apoptotic cells, its measurement can be relevant to other medical conditions such as autoimmune diseases, infectious diseases, pregnancy conditions and trauma.(7) Identifying specific epigenetic signatures related to these conditions can be considered a minimally invasive liquid biopsy to help evaluate therapeutic potential in clinical studies. 

Both plasma and serum contain antibodies, which can be used to develop countermeasures against a particular pathogen or poison, such as in the treatment of bites or stings from venomous animals. Various monoclonal antibodies have been isolated and identified from serum or plasma for a range of therapeutic uses, including treating infectious diseases such as Ebola, anthrax, and COVID-19, as well as inflammatory diseases like rheumatoid arthritis, lupus, or sickle cell disease. 

Since plasma contains anticoagulants that prevent clot formation, it is also often used for blood transfusion in clinical practice. Plasma can be used as a therapeutic treatment for people with chronic health diseases such as liver disease, autoimmune conditions and hemophilia. Evidence from early-phase clinical trials suggests that blood plasma from young donors can also show signs of clinical improvements in early-stage Alzheimer’s patients (8,9), similar to what was first observed in animal studies.(10) Furthermore, recent work in a mouse model demonstrated that the exercise-induced metabolite, N-lactoyl-phenylalanine, can be exogenously administered to mice to decrease body adiposity and weight, uncovering a novel therapeutic avenue that could be harnessed in the treatment of obesity.(11) Mechanistic studies identifying which specific circulating factors can contribute to developing therapeutic approaches are exciting areas of research from cancer to metabolism. 

Utilizing serum and plasma samples for research is particularly attractive given the minimal invasiveness and the wealth of biological information that can be uncovered in healthy and disease states. The research and clinical applications of biomarkers derived from serum and plasma samples continue to expand and can offer valuable insights into understanding and treating various diseases. Knowing which type of sample to collect and minimizing variability in collection techniques within a study and across comparable studies is crucial to generating reliable and reproducible data that can guide drug discovery and clinical approaches.

References

1. Yu Z, Kastenmüller G, He Y, Belcredi P, Möller G, Prehn C, et al. Differences between Human Plasma and Serum Metabolite Profiles. Oresic M, editor. PLoS ONE. 2011 Jul 8;6(7):e21230. 
2. Vignoli A, Tenori L, Morsiani C, Turano P, Capri M, Luchinat C. Serum or Plasma (and Which Plasma), That Is the Question. J Proteome Res. 2022 Apr 1;21(4):1061–72. 
3. Mannello F. Serum or Plasma Samples?: The “Cinderella” Role of Blood Collection Procedures Preanalytical Methodological Issues Influence the Release and Activity of Circulating Matrix Metalloproteinases and Their Tissue Inhibitors, Hampering Diagnostic Trueness and Leading to Misinterpretation. ATVB. 2008 Apr;28(4):611–4. 
4. Wakefield LM, Letterio JJ, Chen T, Danielpour D, Allison RS, Pai LH, et al. Transforming growth factor-beta1 circulates in normal human plasma and is unchanged in advanced metastatic breast cancer. Clin Cancer Res. 1995 Jan;1(1):129–36. 
5. Jeon JH, Kim YS, Choi EJ, Cheon S, Kim S, Kim JS, et al. Implication of co-measured platelet factor 4 in the reliability of the results of the plasma transforming growth factor-beta 1 measurement. Cytokine. 2001 Nov 7;16(3):102–5. 
6. Kong FMS, Zhao L, Wang L, Chen Y, Hu J, Fu X, et al. Ensuring sample quality for blood biomarker studies in clinical trials: a multicenter international study for plasma and serum sample preparation. Transl Lung Cancer Res. 2017 Dec;6(6):625–34. 
7. Tsang JCH, Lo YMD. Circulating nucleic acids in plasma/serum. Pathology. 2007 Apr;39(2):197–207. 
8. Sha SJ, Deutsch GK, Tian L, Richardson K, Coburn M, Gaudioso JL, et al. Safety, Tolerability, and Feasibility of Young Plasma Infusion in the Plasma for Alzheimer Symptom Amelioration Study: A Randomized Clinical Trial. JAMA Neurol. 2019 Jan 1;76(1):35–40. 
9. Boada M, López OL, Olazarán J, Núñez L, Pfeffer M, Paricio M, et al. A randomized, controlled clinical trial of plasma exchange with albumin replacement for Alzheimer’s disease: Primary results of the AMBAR Study. Alzheimer’s & Dementia. 2020 Oct;16(10):1412–25. 
10. Villeda SA, Plambeck KE, Middeldorp J, Castellano JM, Mosher KI, Luo J, et al. Young blood reverses age-related impairments in cognitive function and synaptic plasticity in mice. Nat Med. 2014 Jun;20(6):659–63.
11. Li VL, He Y, Contrepois K, Liu H, Kim JT, Wiggenhorn AL, et al. An exercise-inducible metabolite that suppresses feeding and obesity. Nature. 2022 Jun;606(7915):785–90.