Test Time...How the "Body Clock" Affects Diagnostic Testing The body clock's powerful influence can also be seen in the way it affects diagnostic test results. These results can vary significantly, possibly producing inaccurate readings, depending on the time of day when a test is done. Consider how the body clock affects blood pressure. Blood pressure isn't constant throughout the day and night: it normally rises in the morning, remains elevated during the day and early evening, and decreases to its lowest level during sleep. So a single reading taken during the day may not give a true picture of whether blood pressure is within the normal range or requires treatment. Some doctors now ask patients to wear special monitoring devices. These provide a complete 24-hour blood pressure pattern by recording blood pressures a number of times during the day. The body clock also affects skin testing for allergies. Results are lower in the morning, considerably higher in the evening, and greatest just before bedtime. The body clock may complicate testing for asthma severity. Airway patency, or degree of openness, is poorest during the night and best during midday and evening. The same is generally true for forced expiratory volume and peak expiratory flow readings, which can be up to 50% higher at midday and in the afternoon than overnight or when the patient wakes up. Unless these circadian rhythms are taken into account, daytime evaluation of asthma patients could result in underestimating the severity of the disease. CIRCADIAN RHYTHMS OF DISEASES Peak Time of Event/Variable Michael H. Smolensky, PhD What's being done to help doctors obtain the most accurate results from diagnostic tests likely to be affected by the body clock? Just heightening their awareness of the problem helps. In addition, medical chronobiologists are working to develop test-interpretation guidelines for doctors that take circadian rhythms into account. For additional information about Biologic Rhythms, see these MedicineNet.com articles: Biologic Rhythms Body Rhythms and Angina Body Rhythms and Heart Attack Body Rhythms and Hypertension Body Rhythms and Allergies Body Rhythms and Asthma Biologic Rhythms and Symptoms of Illness Biologic Rhythms and Diagnostic Testing References: The right time? Chronopharmacology - a new science. Nursing RSA Verpleging. 1992;7:23-27. Pharmacology. Smolensky MH, D'Alonzo GE. Medical chronobiology: concepts and applications. Am Rev Respir Dis.1993;147:S2-S 19. Time of day medicine dose is taken may boost its efficacy, cut toxicity. JAMA. 1996;275:1 143-1 144. Medical News & Perspectives.