Research: PFL testing for post-exertional malaise and disability, Research1st website, 18 November 2011

From the Research1st website maintained by the CFIDS Association of America, 18 November 2011.

PFL Testing for Post-Exertional Malaise and Disability

by Staci R Stevens MA and Christopher R Snell, PhD

The Pacific Fatigue Laboratory’s (PFL) two-day testing protocol is designed to objectively measure functional capacity and the symptom of post-exertional malaise in chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME). In the course of our research, we have performed more than 2,000 cardiopulmonary exercise tests (CPETs) for patients diagnosed with CFS/ME[1]. Many of these patients reported prolonged recovery following a single test. Overdoing physical activity by even a small amount invariably resulted in symptom flare up or post-exertional malaise[2].

Our group subsequently developed a testing regimen (Stevens Protocol), requiring patients to perform a second test 24 hours after the first test to assess physical function in the post-exertional state. An inability to reproduce values for oxygen use on the second test at both peak exercise and at the anaerobic threshold provides objective measures of metabolic dysfunction and reduced work efficiency as indicators of post-exertional malaise[3]. CPET performed on consecutive days or weeks is highly reproducible for healthy individuals as well as for those with heart, lung and kidney disease[4]. It is not reproducible for persons with CFS/ME. CPET is invaluable for disability evaluation because it shows high day-to-day variance in the ability to sustain work. The exercise test profiles look like different people on each day and provide the necessary evidence to obtain disability benefits. Armed with this evidence, 90 percent of our clients have received their benefits.

CPET is simply “stress-testing” with some additional complementary measurements. Stress tests use a bicycle or treadmill to look at heart function at rest and during exercise stress. They employ an electrocardiogram (ECG) to monitor heart rhythm with blood pressures taken throughout the test. The CPET simply adds analysis of gases being moved in and out of the mouth throughout the test, achieved using a mask connected to sample tubes that measure every breath during the test. This adds the ability to measure oxygen used by the muscles to produce energy, carbon dioxide exhaled and the amount of air/ventilation moving in and out of the lungs. So, in addition to measuring heart function, cardiopulmonary analysis adds lung and metabolic function, which provide a multisystem view of the body at work.

The two most important measures are peak oxygen use or aerobic capacity (known as VO2peak) and oxygen use at the anaerobic threshold (VO2AT). Work can be sustained below but not above the threshold. CPET is used to measure athletic performance at the high end and for disability assessment at the other end of the functional continuum. In the fields of exercise science and medicine, this approach is considered the gold standard for measuring and evaluating fatigue. Position statements from the American College of Sports Medicine[5], American Heart Association[6], American College of Chest Physicians, American Thoracic Society[7] and the American Medical Association[8], among others, all endorse this method of testing. The PFL has simply adopted this standardized, valid and reliable tool to evaluate post-exertional malaise and disability in CFS/ME. What we find exciting about the two-day Stevens Protocol is that it is being reproduced both nationally and worldwide. Dutch scientist Dr. Ruud Vermeulen[9] has published research replicating our initial findings, and Dr. Betsy Keller at Ithaca College in New York [10] presented similar results at the IACFS/ME conference in September. It is our hope that this protocol will be broadly adopted for CFS/ME to confirm post-exertional malaise, to support disability evaluations and to become the standard for assessing function in research studies and clinical trials.


Vanness, J.M., et al., Subclassifying chronic fatigue syndrome through exercise testing. Med Sci Sports Exerc, 2003. 35(6): p. 908-13.
VanNess, J.M., et al., Postexertional malaise in women with chronic fatigue syndrome. J Womens Health (Larchmt), 2010. 19(2): p. 239-44.
VanNess, J.M., Snell, C.R. and Stevens, S.R. , Diminished Cardiopulmonary Capacity During Post-Exertional Malaise in Chronic Fatigue Syndrome. Journal of Chronic Fatigue Syndrome, 2007. 14(2): p. 77-85.
Hansen, J.E., et al., Reproducibility of cardiopulmonary exercise measurements in patients with pulmonary arterial hypertension. Chest, 2004. 126(3): p. 816-24.
ACSM’s Guidelines for Exercise Testing and Prescription, 8th ed, ed. N.F.G. Walter R. Thompson, Linda S. Pescatello2009, Philadelphia: Lippincott Williams & Wilkins.
Balady, G.J., et al., Clinician’s Guide to cardiopulmonary exercise testing in adults: a scientific statement from the American Heart Association. Circulation, 2010. 122(2): p. 191-225.
ATS/ACCP Statement on cardiopulmonary exercise testing. Am J Respir Crit Care Med, 2003. 167(2): p. 211-77.
Guides to the Evaluation of Permanent Impairment. 5th ed, ed. A.B. Cocchiarella 2001, Chicago: American Medical Association.
Vermeulen, R.C., et al., Patients with chronic fatigue syndrome performed worse than controls in a controlled repeated exercise study despite a normal oxidative phosphorylation capacity. J Transl Med, 2010. 8: p. 93.
Keller, B. Exercise Testing to Quantify Effects of Fatigue on
Functional Capacity in Patients With CFS. in IACFS/ME Biennial Conference; Translating Evidence Into Practice. 2011. Ottawa, Ontario, Canada.

Staci R. Stevens, MA, is the founding executive director of the Pacific Fatigue Laboratory located in the department of sport sciences at the University of the Pacific in Stockton, Calif.

Christopher R. Snell, PhD., is the PFL’s scientific director and professor in the sport sciences department.

For more information on the lab, visit


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