Comparative Analysis of Serum Cytokine ELISA and Multiplex Techniques

Investigation of cytokines in serum analysis has been classically conducted by enzyme-linked immunosorbent assay (ELISA). With an increasing demand for rapid and precise measurements for multiple cytokines and other biomarkers, ELISA has been the standard for quantitative serum analysis. Yet, it is not well suited for high throughput multiple ABSTRACT Enzyme-linked immunosorbent assay (ELISA) has been described as the gold standard for serum cytokine analysis. However, multiplex array technologies are increasing in popularity, likely because of their ability to analyze large numbers of analytes with low sample volume compared to ELISA. Few studies have directly compared serum cytokine results from ELISA and Multiplex analytical techniques. Therefore, the purpose of this observational research was to determine if differences exist in serum cytokine concentrations between ELISA and Multiplex techniques. Blood samples were collected from the antecubital vein of professional American football players during a competitive season to monitor biomarkers fluctuations throughout the season. After clotting, blood was centrifuged and three 300 μL aliquots of serum were frozen at -80°C. IL-1β, IL-6, and TNF-α were measured from the exact same work is needed to determine the cause of these large discrepancies seen between methods.

biomarker detection [5]. Compared to ELISA, multiplex arrays provide quantitative measurement of multiple cytokines on a largescale using bead-based technology. This sought-after assay offers a quantitative measure of multiple cytokines on a large-scale using bead-based technology. Thus, further analysis of these two methods for quantitative serum cytokine analyses is necessary. ELISA has been classically described as the gold standard for serum cytokine analysis [14]. Also known as the sandwich ELISA, an antibody adhered to the bottom of a well provides capturing antigen capability and immune specificity, and a different antibody provides detection and amplification. The method described has been cited as an accurate and sensitive reading of the cytokine of interest [16][17][18][19]. However, the data reading outcome depends on the kit manufacturer, antibody quality, and skill and experience of the operator [14]. On the other hand, the ELISA method provides some limitations. For example, only one cytokine at a time can be measured, limiting the ability of investigators to analyze various cytokines of interest due to limited amount of biological sample available [9,19]. Moreover, ELISA limits the range of cytokine detection, meaning the linear relationship between cytokine concentration and absorbance reading is narrow compared to other technologies such as multiplex arrays.
ELISA samples above the cytokine range require an additional kit for re-testing with a dilution factor creating additional labor, cost, and valuable sample volume, if extra sample is available.
Diluted ELISA values may not be comparable to undiluted ELISA values. This is applicable to serum samples when interpreting the differences in ELISA results between samples that show results within the cytokine range (not requiring dilution) and samples above the cytokine range (requiring dilution) [19]. Multiplex array technologies are increasing in popularity; this is likely due to their ability to simultaneously analyze large numbers of cytokines from a single sample with low volume compared to ELISA [9]. This technology is a flow cytometric (bead-based) assay that allows each bead to be coated with a specific capture antibody. Detection antibodies labeled with a fluorescence compound bind to the specific cytokine-capture antibody complex on each bead. As a result, many cytokines in a biological sample can be detected by the difference in bead fluorescence for nearly 100 biomarkers [19].
When compared to ELISA, multiplex arrays offer these advantages: a) high throughput multiplex analysis, b) reduced sample volume, c) time and cost efficiency, d) ability to evaluate the levels of one given inflammatory biomarker in the context of multiple others, e) ability to perform repeated measures of the same cytokine panels in the same subjects under the same experimental assay conditions, f) ability to reliably detect different proteins across a broad dynamic range of concentrations [19]. The technology of choice is under the discretion of the investigators and depends on the research objective and the biological sample of concern. The comparison between ELISA and Multiplex arrays are acceptable when using tissue culture supernatant samples; however, the comparability is less promising when using serum or plasma samples [13]. Multiplex array technologies are more sensitive than ELISA to alterations in circulating proteins and inhibitors present in serum or plasma, which can complicate levels detected during disease, aging, or inflammatory conditions associated with exercise/sport. IL-1β, IL-6, and TNF-α were selected as the biomarkers of interest to assess the degree of inflammation and muscle damage of professional American football players throughout a season.
These biomarkers have been evaluated and monitored in other sports to determine recovery and inflammation in athletes [1,8,11,12]. Few studies have directly compared serum cytokine results from ELISA and Multiplex analytical techniques, specifically within healthy professional American football players. Therefore, the purpose of this study was to determine differences in serum cytokine concentrations between ELISA and Multiplex techniques.

Statistics
Coefficients of variation (CV%), paired-samples t-test with 95% CI, and Pearson's product-moment correlations were used to compare cytokine analysis techniques, similar to other studies investigating differences between competing methods [4,7].
Descriptive statistics are shown as mean ± SD. Statistical analyses were performed using Minitab software (Minitab Inc., State College, PA).

Results
Serum cytokine concentrations were not comparable between ELISA and Multiplex analytical techniques. ELISA kits did not include internal quality controls. All standard curves performed as expected for each assay (r 2 >0.99). Descriptive statistical

comparison between ELISA and Magpix multiplex for IL-1β
(n=15) displayed significant variability between techniques with a CV% of 119.9 (Table 1). IL-6 (n=33) demonstrated substantial variability between techniques with a CV% of 126.9 (Table 2). For TNF-α (n=36) all ELISA samples were below the detection limit of 0.31 pg/mL with the Magpix Multiplex detecting 8.8 ± 3.2 pg/ mL (intra-assay CV%= 7.9). Due to limited sample availability not all serum samples were able to be performed across all kits.
Additionally, due to no detection of TNF-α from the ELISA kit, no statistical comparison could be determined between ELSIA and Multiplex. Paired samples t-tests showed considerable mean differences between analytical techniques for IL-1β and IL-6 ( Table   3)

Discussion
Cytokines are secreted to combat inflammation and are necessary to respond to invading pathogens. Cytokine measurement and detection in serum are essential for immune health, and the ELISA technique has long been the gold standard for analyzing cytokines in serum [14]. However, with advances in biochemical capabilities and technology, multiplex arrays offer quantitative measurement of multiple cytokines to compare to, from a single sample, unlike ELISA. The comparability has yet to be revealed for quantitative serum cytokine analyses with these two methods.
Therefore, we sought to determine the differences in serum cytokine concentrations between ELISA and Multiplex techniques.
The results presented in this study demonstrate the importance of understanding the limitations and discrepancies between competing analytical methods when designing, conducting, and interpreting research conclusions that involve serum cytokines.
ELISA is limited to the measurement of one cytokine at a time, which restricts the ability to measure multiple biomarkers from a single biological sample [9,19].
This restriction can influence an investigator's ability to answer research questions based on the sample amount available, which was the case in this observational experiment. When samples are above the cytokine range, they are typically diluted and need to be re-analyzed using another assay kit. Dilutions and remeasurements can only occur if there is sample remaining and a diluted sample may reduce the cytokine concentration and diminish any circulating inhibitors or binding protein concentrations [19]. Additionally, freeze-thaw cycles, which are common in multiple ELISAs, may also affect cytokine concentrations. Most cytokines keep their stability for up to 2 years at -80° C, and cytokines such as IL-1β and IL-6 are degraded up to 50% within 2-3 years of storage [2]. Three freezethaw cycles are enough for most cytokines to keep stable [17] however, cytokines such as TNF-α may increase in concentration after each freeze-thaw period showing significant increases after three cycles [6].
ELISA techniques also lack accuracy due to insufficient blocking of immobilized antigen, specifically the hydrophobic binding of immunoglobulin in sample specimens to plastic surfaces. Research has revealed the importance of eliminating any non-specific binding in an immunological assay to avoid false positives and negative results using heterophilic blocking reagents [3,[11][12][13][14][15][16][17][18]. Unfortunately, assay manufacturers do not disclose the components of their blocking agents due to proprietary information. The multiplex assay used in this study contained heterophilic blocking agents, per the manufacturer, but no further information could be given. Per our request, the manufacturers of the ELISA kits did not have any information available on their blocking agents. ELISA lacks a dynamic detection range, can be labor-intensive, expensive, and time consuming compared to Multiplex. Multiplex methodology allows for the simultaneous detection of up to 100 biomarkers from a single low volume sample, which saves time and cost.
Multiplex has the capability to perform repeated measurements of the same biomarkers from the same subjects under the same experimental assay conditions as well as the ability to reliably detect multiple biomarkers across a broad range of concentrations.

Multiplex assays involve interactions between multiple cytokines
in the assay solution, as a result, cross-reactivity may occur [19].
Complications may happen when rare biomarkers are assayed together. Proteins in the serum may also affect the results considering that all multiplex arrays occur in a solution containing freely moving molecules and antigens [19] which is an indicator that multiplex arrays are more sensitive than ELISAs to circulating proteins and inhibitors. While our research suggests that the multiplex and ELISA methods may not be comparable, additional research is warranted across a broader population due to this studies specific and small sample size with healthy and young American football players.
Athletes exhibit fewer changes in cytokine levels following a longer duration of exercise than non-athletes, suggesting that athletes have a reduced cytokine response [8]. A lack of dynamic cytokine range from our subjects is a significant limitation of our work. Additionally, more research is needed to determine the cause of these large discrepancies seen between ELISA and

Conclusion
Navigating the research outcomes across multiple publications has become increasingly difficult for the scientific community to entrust and advance their work due to the lack of comparability across assay kits [15]. The research conclusions from this study confirm those concerns. ELISA has been described as the gold standard for serum cytokine analysis, as well as several other biomarkers. However, the findings from this study advocate further research to determine the source of discrepancies between assay techniques, there is a need for transparency on the types of blocking agents used, as well as the development of validation guidelines which would allow researchers the ability to compare results across multiple techniques and assay manufacturers. The creation of standardized assay guidelines would enhance the interpretation and translation of biomarker research.
Limitations of this study include small sample size, healthy and unique subject population, lack of manufacturer provided controls for the ELISA assays, lack of reference samples analyzed across kits, and the inability to match blocking agents across techniques.