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== Blood tests == === Activin B === [[File:Blood-test-tubes.jpg|alt=Hand holding tubes choosing blood samples |thumb|300x300px|Blood tests are being developed to diagnose ME/CFS]] A 2017 study that used the [[Canadian Consensus Criteria]] (CCC) concludes: "Elevated [[activin B]] levels together with normal [[activin A]] levels identified patients with the diagnostic symptoms of [[CFS/ME]], thus providing a novel serum based test. The [[activin]]s have multiple physiological roles and capture the diverse array of symptoms experienced by CFS/ME patients."<ref name="Lidbury, 2017" /> The same group later used a weighted standing test in order to rate disease severity, and then compared subjects' health status to their levels of activin B. Activin B was once again found to separate people with ME/CFS from healthy controls.<ref>{{Cite journal | last = Richardson | first = Alice M. | last2 = Lewis | first2 = Don P. | last3 = Kita | first3 = Badia | last4 = Ludlow | first4 = Helen | last5 = Groome | first5 = Nigel P. | last6 = Hedger | first6 = Mark P. | last7 = de Kretser | first7 = David M. | last8 = Lidbury | first8 = Brett A. | date = 2018-04-12 | title = Weighting of orthostatic intolerance time measurements with standing difficulty score stratifies ME/CFS symptom severity and analyte detection | url =https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5898049/|journal=Journal of Translational Medicine|volume=16|doi=10.1186/s12967-018-1473-z|issn=1479-5876|pmc=5898049|pmid=29650052}}</ref> === Buspirone challenge test === {{main article| page_name =Buspirone challenge test}} The [[buspirone challenge test]] measures how much prolactin is released into the bloodstream when a single dose of the drug [[buspirone]] (a 5-HT1A [[serotonin]] receptor agonist) is orally administered. This test has been shown to distinguish ME/CFS patients from healthy controls, as well as being able to distinguish ME/CFS patients from patients with depression. ===CellTrend diagnostic test=== CellTrend bases its test on the theory that a subset of 20-30% of all patients suffering from ME/CFS have developed elevated levels of three [[autoantibody|auto-antibodies]], i.e., auto-antibodies against the b2-adrenergic receptor, auto-antibodies against the [[muscarinic cholinergic receptor|muscarinic cholinergic receptor 3]] (M3) and auto-antibodies against the [[muscarinic cholinergic receptor 4]] (M4).<ref>{{Cite web | url = https://www.celltrend.de/en/pots-cfs-me-crps.html | title = POTS - CFS/ME - CRPS|website=CellTrend Luckenwalde|language=en-US|access-date=2020-02-04}}</ref><ref name="Loebel, 2016" /> Obstacle for use: Only detects a subset 20-30%. Not widely accepted. Patients pay the test cost. ===Cytokine expression=== ''{{main article| page_name=Cytokine}}'' Several researchers are exploring if [[cytokine]] expression in ME/CFS is a unique enough signature to be used as a diagnostic marker.<ref name="Russell, 2016" /><ref name="Peterson, 2015" /><ref name="Hornig, 2016" /><ref name="Montoya, 2017" /> Cytokine expression changes in ME/CFS related to length of illness, with some cytokines levels increasing and some decreasing dependent on illness duration. [[Lindsey Russell|Russell]], et al, focused on a subset of three cytokines, [[IL-1α]], [[Interleukin 6|6]] and [[Interleukin 8|8]], in plasma samples and concluded that: "Setting these 3 markers as a triple screen and adjusting their contribution according to illness duration [the] sub-groups produced ME/CFS classification accuracies of 75–88%."<ref name="Russell, 2016" /> Obstacle for use: Since cytokine expression changes in ME/CFS related to progression of the illness, the validity of potentially useful markers may be obscured by such variation.<ref name="Russell, 2016" /> In 2016, Landi, et al, "observed highly significant reductions in the concentration of circulating [[interleukin]] [[Interleukin 16|(IL)-16]], [[Interleukin 7|IL-7]], and [[Vascular endothelial growth factor|Vascular Endothelial Growth Factor]] A (VEGF-A) in ME/CFS patients" but not in patients of other chronic illnesses with fatigue as a symptom. This three cytokine pattern is being studied further as a potential biomarker for ME/CFS.<ref name="Landi, 2016" /> In 2018, [[Kegan Moneghetti|Moneghetti]], et al, compared the results of cytokine profiles 18 hours post exercise for ME/CFS patients vs healthy patients matched for cardiac structure and exercise capacity. They found that the most discriminatory cytokines detected post exercise in ME/CFS patients were [[CD40L]], [[platelet activator inhibitor]], [[Interleukin 1 beta|interleukin 1-β]], [[Interferon alpha|interferon-α]] and [[CXCL1]]. They concluded that cytokine profiling following exercise may help differentiate patients with ME/CFS from sedentary controls.<ref name="Moneghetti, 2018" /> Obstacle for use: These studies need to be duplicated with other patient cohorts to assure the results are specific enough for an accurate diagnosis of ME/CFS. ===Dysfunction of TCA and urea cycles=== A 2016 study in [[Japan]], by [[Emi Yamano|Yamano]], et al, looked at the differences in intermediate metabolite concentrations in the [[Citric acid cycle|tricarboxylic acid]] (TCA) and [[urea cycle]]s in CFS patients versus healthy controls: "CFS patients exhibited significant differences in intermediate [[metabolite]] concentrations in the [[Citric acid cycle|tricarboxylic acid]] (TCA) and urea cycles. The combination of [[ornithine]]/[[citrulline]] and [[Pyruvate dehydrogenase|pyruvate]]/[[isocitrate]] ratios discriminated CFS patients from healthy controls, yielding area under the receiver operating characteristic curve values of 0.801 (95% confidential interval [CI]: 0.711–0.890, P < 0.0001) and 0.750 (95% CI: 0.584–0.916, P = 0.0069) for training (n = 93) and validation (n = 40) datasets, respectively. These findings provide compelling evidence that a clinical diagnostic tool could be developed for CFS based on the ratios of metabolites in plasma."<ref name="Yamano, 2016" /> Obstacle for use: The specialized laboratory equipment needed for this test is usually only available in facilities engaged in research. The findings in this published study need to be validated through replication by other studies. ===EBV-encoded DNA polymerase and EBV-encoded dUTPase=== In 2012, Dr [[Ronald Glaser]] and Dr [[A Martin Lerner]], et al, discovered that the proteins, [[Epstein-Barr virus]]-encoded DNA polymerase and [[Epstein-Barr virus]]-encoded [[dUTPase]], were present in the blood sera of an [[Epstein-Barr virus|EBV]] subset of [[Chronic fatigue syndrome|CFS]] patients, but was negative in sera of controls.<ref name="Lerner, 2012" /> Obstacle for use: These were preliminary findings that need to be are corroborated by studies with a larger number of [[Epstein-Barr virus|EBV]] subset CFS patients. ===Electrical Impedance=== Experimentation is also being carried out at the [[End ME/CFS Project]] with the [[Open Medicine Foundation]] to test if a [[metabolic]] testing device using new nanofabricated technology that measures electrical impedance could be used to develop a simple diagnostic test for (ME/CFS). Blood from patients with ME/CFS causes a rapid and significant rise in electrical impedance when placed in this device, whereas blood from healthy people does not. The device is inexpensive and gives a real-time assay. This nanofabricated technology has the potential to be place in a hand-held device and disseminate to physician's offices for use as a test during medical visits.<ref>{{Citation | title = An Update on ME/CFS Research with Dr. Ronald W. Davis |url =https://www.youtube.com/watch?v=sGBXXlQO49g|language=en|access-date=2020-02-04}}</ref> A study on this was published in 2019, and described the nanoneedle designed for the test.<ref name="Davis2019" /> Obstacle for use: The technology is still very new and needs further testing. If the technology works, development into an inexpensive, easily disseminated, handheld device is necessary. ===Extracellular Vesicles=== A 2018 study from [[Spain]] reported that [[extracellular vesicle]]s (EVs) were significantly smaller as measured by protein cargo, size distribution and concentration, and the amount of EV-enriched fraction was significantly higher in the blood serum of CFS/ME patients versus healthy controls. Extracellular vesicles are secreted from both healthy cells and cells undergoing apoptosis (i.e., cell death) and play a role in intercellular communication.<ref name="Castro-Marrero, 2018" /> Obstacle for use: The study population consisted of 10 Spanish CFS/ME patients and 5 matched healthy controls. This study would need to be reproduced in larger populations to verify statistic significance. ===Growth Differentiation Factor 15=== GDF15 has been proposed as a biomarker for fatigue severity in ME/CFS patients.<ref name="Melvin2019">{{Cite journal | last = Melvin | first = Audrey | authorlink = Audrey Melvin | last2 = Lacerda | first2 = Eliana | authorlink2 = Eliana Lacerda | last3 = Dockrell | first3 = Hazel | authorlink3 = Hazel Dockrell | last4 = O'Rahilly | first4 = Stephen | authorlink4 = Stephen O'Rahilly | last5 = Nacul | first5 = Luis | author-link5 = Luis Nacul | date = 2019-11-27 | title = Circulating levels of GDF15 in patients with Myalgic Encephalomyelitis/Chronic Fatigue Syndrome | url =https://www.repository.cam.ac.uk/handle/1810/299333|journal=Journal of Translational Medicine|language=en|volume=|issue=| pages=|doi=10.17863/CAM.46401|issn=1479-5876|pmc=|pmid=|access-date=Dec 1, 2019|quote=|via=}}</ref> Melvin et al (2019) used samples from the [[UK ME/CFS biobank]] to compare ME/CFS patients, and found fatigue severity positively correlated with severity of symptoms, and that higher GDF15 levels were found in ME/CFS patients, including mild/moderate patients, in comparison to either [[multiple sclerosis]] patients or healthy controls. The study used samples from 50 severe ME/CFS patients and 100 mild/moderate ME/CFS patients.<ref name="Melvin2019" /> Results have not yet been replicated. ===Immunosignatures=== [[Immunosignaturing]] is a medical diagnostic test which uses arrays of random-sequence peptides (i.e., linked amino acids) to measure specific antibodies circulating in the blood. The [[Nevada Center for Biomedical Research]] in Reno, Nevada, [[United States|US]] has identified a unique Immunosignature (IMS) comprised of a subset of 25 peptides that differentiates the blood serum from people with ME compared to controls with 92.9% specificity and 97.6% sensitivity. [[Myalgic encephalomyelitis|ME]] patients met both the [[International Consensus Criteria]] (CCC) and [[Fukuda criteria]].<ref name="Singh, 2016" /> === Messenger RNA (mRNA) === In 2021, a Dutch study identified a signature of 23 genes capable of distinguishing between ME/CFS cases and healthy controls. They used publicly available mRNA expression data and and DNA methylation data from peripheral blood mononuclear cells (PBMCs) of 93 ME/CFS patients and 25 healthy controls. The authors believe that ten of the 23 genes could be interpreted in the context of the derailed immune system of ME/CFS. The ten proteins focused on in this study are MAPK4, ARRB1, GOLGA4, ABCE1, PHKA2, IL2RB, CCR4, HLA-DQA1, PRG4 and OGG1. The mRNA transcripts encoding these proteins were all downregulated in ME/CFS patients compared to healthy controls.<ref>{{Cite journal | last = Metselaar | first = Paula I. | authorlink = | last2 = Mendoza-Maldonado | first2 = Lucero | authorlink2 = | last3 = Li Yim | first3 = Andrew Yung Fong | authorlink3 = | last4 = Abarkan | first4 = Ilias | authorlink4 = | last5 = Henneman | first5 = Peter | author-link5 = | last6 = te Velde | first6 = Anje A. | author-link6 = | last7 = Schönhuth | first7 = Alexander | last8 = Bosch | first8 = Jos A. | last9 = Kraneveld | first9 = Aletta D. | date = Feb 2021 | title = Recursive ensemble feature selection provides a robust mRNA expression signature for myalgic encephalomyelitis/chronic fatigue syndrome | url =http://www.nature.com/articles/s41598-021-83660-9|journal=Scientific Reports|language=en|volume=11|issue=1 | page = 4541 |doi=10.1038/s41598-021-83660-9|issn=2045-2322|pmc = 7907358|pmid=33633136|access-date=|quote=|via=}}</ref> ===Metabolomics=== ''{{main article| page_name =Metabolomics}}'' [[Robert Naviaux |Naviaux]], et al, at the University of California, San Diego School of Medicine reported in 2016 "that targeted, broad-spectrum [[metabolomics]] of plasma not only revealed a characteristic chemical signature but also revealed an unexpected underlying biology." The researchers found that CFS patients had an average of 40 metabolic abnormalities. From these, a set of 8 metabolites were identified in male patients and a set of 13 metabolites in females that performed well together as a diagnostic test. Both sets gave a 95% Confidence Interval. The eight metabolites selected for males were [[phosphatidyl choline PC]](16:0/16:0), [[glucosylceramide GC]](18:1/16:0), [[1-P5C]], [[FAD]], [[pyroglutamic acid]] (also known as [[5-oxoproline]]), [[2-hydroxyisocaproic acid]] (HICA), [[serine|l-serine]], and [[lathosterol]]. The 13 metabolites selected for females were [[THC]](18:1/24:0), phosphatidyl choline PC(16:0/16:0), [[hydroxyproline]], [[ceramide]](d18:1/22:2), [[lathosterol]], [[Adenosine triphosphate|adenosine]], [[phosphatidylinositol PI]](16:0/16:0), FAD, 2-octenoylcarnitine, [[phosphatidyl choline plasmalogen PC]](22:6/P18:0), phosphatidyl choline PC(18:1/22:6), 1-P5C, and [[CDCA]]. There are several metabolites that appear on both the males and female lists.<ref name="Naviaux, 2016" /> Obstacle for use: The specialized laboratory equipment needed for this test is usually only available in facilities engaged in research. ===MicroRNA (miRNA)=== [[microRNA]] (miRNA) are molecules involved in [[gene expression]]. (Note:MicroRNA are different from [[Messenger RNA|mRNA]] which stands for messenger RNA.) A 2016 study by Petty, et al found that "four upregulated miRNA were suitable markers to resolve [[CFS/ME]] subjects from a matched control cohort."<ref name="Petty, 2016" /> A 2012 study by [https://www.ncbi.nlm.nih.gov/pubmed/22572093 Brenu, et al] found the expression of eight specific miRNAs "significantly decreased in NK cells of CFS/ME patients in comparison to the non-fatigued controls" and one specific miRNA was significantly downregulated in "both the NK and CD8(+)T cells in the CFS/ME sufferers."<ref name="Brenu, 2012" /> In 2015, [[Griffith University]] filed for a patent for a biological marker (Patent Publication number WO2016023077 A1) for the diagnosis and management of ME and CFS. [[Sonya Marshall-Gradisnik]] and [[Ekua Brenu]] are listed as the inventors. The patent application states: "The present invention resides broadly in the use of at least one miRNA as a biological marker for identifying or diagnosing a subject having CFS and/or ME."<ref>{{Citation | title = Biological markers |url =https://patents.google.com/patent/WO2016023077A1/en|language=en|access-date=2020-02-04}}</ref> In 2016, Griffith University's Professor [[Donald Staines]] and Professor [[Sonya Marshall-Gradisnik]] announced that they have been awarded a $4-million grant to be administered during the next five years that will enable them to continue research into developing a diagnostic test for ME/CFS.<ref>{{Cite web | url = https://www.goldcoastbulletin.com.au/lifestyle/gold-coast-team-getting-closer-to-discovering-test-for-chronic-fatigue-syndrom/news-story/1627b3fcc41c53efac927200a53cc756 | title = Coast team zeroing in on CFS test | date = 2016-12-01 | website = goldcoastbulletin.com.au|language=en|access-date=2020-02-04}}</ref> Almenar-Pérez et al (2020) analyzed microRNAs from both [[peripheral blood mononuclear cell]]s and extracellular vesicles in 15 severely ill myalgic encephalomyelitis/<wbr>chronic fatigue syndrome patients, finding underexpression of [[AGO2]] and [[MECP2]] and elevated levels of [[NTRK#NTRK1|NTRK1]].<ref name="AlmenarPerez2020">{{Cite journal | last = Almenar-Pérez|first = Eloy | authorlink = | last2 = Sarría | first2 = Leonor | authorlink2 = | last3 = Nathanson | first3 = Lubov | authorlink3 = Lubov Nathanson | last4 = Oltra | first4 = Elisa | authorlink4 = Elisa Oltra | date = 2020-02-07 | title = Assessing diagnostic value of microRNAs from peripheral blood mononuclear cells and extracellular vesicles in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome | url =https://www.nature.com/articles/s41598-020-58506-5|journal=Scientific Reports|language=en|volume=10|issue=1 | pages = 1–15|doi=10.1038/s41598-020-58506-5|issn=2045-2322|pmc=|pmid=|access-date=Feb 12, 2020|quote=|via=}}</ref> Although the science of using the detection of different miRNAs for disease confirmation is still in its infancy. There are no established baseline data for miRNAs among normal individuals, which would be necessary for using miRNA levels as biomarkers.<ref name="Tonge, 2016" /> The findings in the studies published in March 2016 and January 2020 need to be validated through replication by other studies. The 2020 involved only severely ill women, and would need to be validated for use in children, men and those who are not severely ill. The [[Griffith University]] test has not been released for public use to date. === Mitochondrial energy production blockage === Studies by Myhill, Booth and McLaren-Howard<ref>{{Cite journal | last = Myhill|first = Sarah | last2 = Booth | first2 = Norman E. | last3 = McLaren-Howard | first3 = John | date = 2009-01-15 | title = Chronic fatigue syndrome and mitochondrial dysfunction | url =https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2680051/|journal=International Journal of Clinical and Experimental Medicine|volume=2|issue=1 | pages = 1–16|issn=1940-5901|pmc=2680051|pmid=19436827}}</ref><ref>{{Cite journal | last = Booth|first = Norman E | last2 = Myhill | first2 = Sarah | last3 = McLaren-Howard | first3 = John | date = 2012-06-15 | title = Mitochondrial dysfunction and the pathophysiology of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) | url = https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3403556/|journal=International Journal of Clinical and Experimental Medicine|volume=5|issue=3 | pages = 208–220|issn=1940-5901|pmc=3403556|pmid=22837795}}</ref><ref>{{Cite journal | last = Myhill|first = Sarah | last2 = Booth | first2 = Norman E | last3 = McLaren-Howard | first3 = John | date = 2012-11-20 | title = Targeting mitochondrial dysfunction in the treatment of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) - a clinical audit | url = https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3515971/|journal=International Journal of Clinical and Experimental Medicine|volume=6|issue=1 | pages = 1–15|issn=1940-5901|pmc=3515971|pmid=23236553}}</ref> have shown the mitochondria of ME/CFS patients to have energy production defects. Their studies employed the ATP Profiles Test<ref>{{Cite web | url = https://www.drmyhill.co.uk/wiki/Mitochondrial_Function_Profile | title = Mitochondrial Function Profile - DoctorMyhill | website = drmyhill.co.uk|access-date=2020-02-04}}</ref> (Acumen Laboratory, Tiverton, Devon, UK) to measure the functional efficiency of five metabolic processes involved in mitochondrial energy production (the test provides five numerical values indicating the functional efficiency of each energy metabolism process). Using this ATP Profiles Test, the authors discovered that almost all the 200 or so ME/CFS patients in their cohorts had impaired mitochondrial energy production. However, out of the five energy metabolism processes measured, each patient had their own particular processes that were at fault (running at low efficiency), and their own particular processes which were working OK (running at normal efficiency). Because these studies discovered ME/CFS patients can have energy metabolism defects in several of the energy metabolism processes measured, the authors combined the efficiency values for each of the 5 processes into one single efficiency figure, which they call the Mitochondrial Energy Score (MES). The MES is thus a single numerical value that gives the overall efficiency of a patient's mitochondrial energy production. The authors found there is a high degree of correlation between the Mitochondrial Energy Score and the degree of severity of ME/CFS (severity as measured on the [[Bell CFIDS disability scale|Bell scale]]). Furthermore, the Mitochondrial Energy Score was able to successfully distinguish between ME/CFS patients and healthy controls in nearly all cases. ===Nanoelectronics-blood-based diagnostic biomarker=== A nanoelectronics-based biomarker for [[ME/CFS]] was found by a team lead by [[Ron Davis]], at [[Stanford University]], in 2019. This blood test uses electrical impedance testing and correctly identified 100% of patients with ME/CFS, diagnosed according to the [[Canadian Consensus Criteria]].<ref name="Davis2019">{{Cite journal | last = Davis | first = R. W. | authorlink = Ronald Davis | last2 = Wilhelmy | first2 = J. | authorlink2 = Julie Wilhelmy | last3 = Nemat-Gorgani | first3 = M. | authorlink3 = Mohsen Nemat-Gorgani | last4 = Kashi | first4 = A. | authorlink4 = Alex Kashi | last5 = Esfandyarpour | first5 = R. | author-link5 = Rahim Esfandyarpour | date = Apr 25, 2019 | title = A nanoelectronics-blood-based diagnostic biomarker for myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) | url = https://www.pnas.org/content/early/2019/04/24/1901274116|journal=Proceedings of the National Academy of Sciences|language=en|volume=116|issue=21 | pages = 10250-10257|doi=10.1073/pnas.1901274116|issn=0027-8424|pmid=31036648|quote=|via=}}</ref> Only 20 patients gave samples for this study, and 20 controls.<ref name="Davis2019" /> The blood test involves putting the [[leukocyte|white blood cell]] samples under physical stress, and relied on nanotechnology using a specially designed nanoneedle.<ref name="Davis2019" /> {{See also|A nanoelectronics-blood-based diagnostic biomarker for myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS)}} ===Natural killer cell function=== Numerous studies of CFS have found evidence of reduced [[natural killer cell]] (NK) function.<ref name="Barker1994" /><ref name="WhitesideTL1998" /><ref name="BrenuEW2014" /><ref name="FletcherMA2002" /><ref name="BrenuEW2012" /> Some studies have showed NK function correlates with illness severity.<ref name="OjoAmaize1994" /> One study found increased differentiation in NK cells.<ref name="HuthTK2016" /> Obstacles for use: Specialized lab equipment not available in average laboratories; blood specimen must be tested within 48 hours of draw and must remain at between 59°-98.6°F or 15°-37°C so special considerations are needed in transporting blood specimens to specialized labs. Reduced NK cell function can be seen with other immune-related conditions,<ref>Orange, J. S. (2013). Natural killer cell deficiency. The Journal of Allergy and Clinical Immunology, 132(3), 515–526. http://doi.org/10.1016/j.jaci.2013.07.020</ref> so this test would need to be paired with other clinical or laboratory findings to make a definitive diagnosis. === Phenylalanine measured via Raman microspectroscopy === A research team (Xu, et al, 2018) in the [[United Kingdom|UK]] used [[single-cell Raman microspectroscopy]] (SCRM) on rho zero cells (ρ<sup>0</sup> cells) that lack [[mitochondrial DNA]] (mtDNA) compared to [[peripheral blood mononuclear cell]]s (PBMCs) from CFS patients and healthy controls. They found that Raman [[phenylalanine]] bands associated with CFS patient PBMCs were significantly higher and were increased in intensities compared to controls. The results suggest that the increase in cellular phenylalanine may relate to [[mitochondrion|mitochondrial]]/energetic dysfunction in CFS and that phenylalanine can be used as a potential biomarker for diagnosis of CFS by SCRM. They achieved an accuracy rate of 98% correctly determining between CFS patients and healthy controls.<ref>{{Cite journal | last = Xu|first = Jiabao | last2 = Potter | first2 = Michelle | last3 = Tomas | first3 = Cara | authorlink3 = Cara Tomas | last4 = Elson | first4 = Jo | last5 = Morten | first5 = Karl | author-link5 = Karl Morten | last6 = Poulton | first6 = Joanna | last7 = Wang | first7 = Ning | last8 = Jin | first8 = Hanqing | last9 = Hou | first9 = Zhaoxu | date = 2018 | title = A new approach to find biomarkers in chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) by single-cell Raman micro-spectroscopy | url = https://pubs.rsc.org/en/content/articlelanding/2018/an/c8an01437j/unauth#!divRelatedContent&articles|journal=The Analyst|language=en|volume=| pages=|doi=10.1039/C8AN01437J|issn=0003-2654|via=}}</ref> Obstacles in use: Further testing needed for confirmation. ===Plasma neuropeptide Y=== Plasma levels of neuropeptide Y (NPY), a neurotransmitter in high quantity in the [[brain]], are reported to be elevated in complex multi-symptom illnesses associated with [[immune system|immunologic] dysfunction. [[Mary Ann Fletcher| Fletcher]], et al, did a study believed to be "the first in the CFS literature to report that plasma NPY is elevated [in ME/CFS] compared to healthy controls and to a fatigued comparison group, GWI [Gulf War Illness] patients. The significant correlations of NPY with stress, negative mood, general health, depression and cognitive function strongly suggest that this peptide be considered as a biomarker to distinguish subsets of CFS."<ref name=":0">{{Cite journal | last1 = Fletcher | first1 = Mary Ann | authorlink = Mary Ann Fletcher | last2 = Rosenthal | first2 = Martin | last3 = Antoni | first3 = Michael | authorlink3 =Michael Antoni | last4 = Ironson | first4 = Gail | last5 = Zeng | first5 = Xiao R | last6 = Barnes | first6 = Zachary | author-link6 = Zachary Barnes | last7 = Harvey | first7 = Jeanna M | author-link8 = Jeanna Harvey | last8 = Hurwitz | first8 = Barry | last9 = Levis | first9 = Silvina | last10 = Broderick | first10 = Gordon | author-link10 = Gordon Broderick | last11 = Klimas | first11 = Nancy G | author-link11 = Nancy Klimas | title = Plasma neuropeptide Y: a biomarker for symptom severity in chronic fatigue syndrome|journal = Behavioral and Brain Functions| volume = 6 | issue = 76| pages = | date =2010|doi= 10.1186/1744-9081-6-76}}</ref> Obstacles in use: NPY is elevated in other immune illnesses such as [[rheumatoid arthritis]] (RA) and [[systemic lupus erythematosus]] (SLE). NPY varies greatly between individuals and is interdependent on other cellular and molecular components and must be viewed with data from other biomarkers.<ref name=":0" /> ===Ratio of active:inactive PKR=== In 2018, [[Eiren Sweetman]] discovered, during work for her doctorate thesis, that a changed ratio of active:inactive [[Protein Kinase R]] (PKR) in people with ME/CFS vs age-gender matched controls could potentially be used as a diagnostic biomarker. Protein Kinase R (PKR) is an innate antiviral immune response protein. Upon measuring an antibody against a non-phosphorylated PKR fragment and an antibody against a [[Thr-446]] phosphorylated PKR peptide, an increased ratio of phosphorylated PKR to non-phosphorylated inactive PKR was detected ME/CFS patients.<ref name="Sweetman2018">Sweetman, E. C. (2018). Comprehensive molecular analysis of different classes of molecules in a Myalgic Encephalomyelitis/Chronic Fatigue Syndrome pilot study group, and investigation of RNA-activated Protein Kinase R (PKR) as a diagnostic biomarker (Thesis, Doctor of Philosophy). University of Otago. Retrieved from http://hdl.handle.net/10523/7834</ref> Obstacles in use: Further testing needed for confirmation; test would need to be adapted from a research setting to a clinical setting for wide availability.
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