Biomarkers are very important in gray areas where diseases have overlapping symptoms or core symptoms in common. Currently the scientific literature is muddied by multiple symptom based definitions making extrapolation from cohorts defined by one definition to all patients. For example a recent study by UK psychiatrists found that the cohort they studied using a rarely used definition that did not include post exertional malaise would have been strengthened had they used biomarkers or even the standard criteria to define patients.