It's about time: a chronobiological approach to healthcare.

Are you a morning rather than an evening person? Does a PM shift assignment stretch your coping abilities to the maximum but seem ideally suited to the energies of your coworkers? Have you ever wondered about the clinical significance of daily rhythmic variations in serum analyte concentrations?

The bases for these observations are being investigated by researchers in the field of chronobiology -- the study of biological rhythms and trends.

AS OLD AS TIME

The rhythms of the human body and their relationship to predictable environmental cycles have been noted for centuries. Ancient healers routinely admonished that, to be successful, treatment had to be provided with strict regard for various internal and external cycles.

Modem recognition of the importance of understanding the human biological clock and its implications for medicine dates back to the 1950s and 1960s. Contemporary research is confirming and validating these earlier observations.

Application of the principles of chronobiology may help in disease prevention, aid in early diagnosis and treatment, and reduce overall health care costs.[1,2] We already know the importance of specimen-collection time in evaluating the clinical significance of certain lab results. Time also figures into maximizing employee productivity on all shifts as well as optimizing patient response to treatment.

Rhythmic changes with different frequencies are reported to occur in a variety of biochemical, hematologic, endocrine, immune, and behavioral activities.[3,4,5,6] By relating internal and external schedules, chronobiology allows exploration of variations in physiologic and pathophysiologic mechanisms over time. Chronotherapy, whether by chemical or physical agent, is concerned with achieving maximum benefit for the patient by providing treatment according to the body's time structure. Chronopharmacology ties drug therapy to rhythmic biologic alterations and looks at response in relation to time of medication administration.

THE LANGUAGE OF RHYTHMS

At all levels of the organization of life, from the cellular to the organismic, functions fluctuate rhythmically. Most rhythms, though responsive to external stimuli, are genetically based. Even when environmental and social prompts are closely controlled, endogenous fluctuations will persist. They may, however, become "free running" -- that is, the rhythms will gradually drift out of synchrony with the environment. Rhythmic events interact with each other and the synchronization of multiple parameters results. The desynchronization of biorhythms increases the risk of disease or dysfunction. Pattern abnormalities, trend shifts, or loss of timing may also presage the occurrence of pathology.

The most thoroughly researched biorhythm is the circadian ["circa" (about) and "dies" (day)] pattern, which is based on a 20-28 hour interval. A cycle including a shorter interval, one less than 20 hours, is referred to as ultradian; infradian rhythms have a "repeat" that is greater than 28 hours. Circaseptan (about weekly) and circannual (about yearly) rhythms are examples of infradian rhythms. To help detect a rhythmic phenomenon, we prepare a chronogram in which we plot data as a function of time. We can evaluate the data by statistical procedures, e.g., analysis of variance or t-test, to determine if a time-dependent variation exists. Further statistical data analysis provides information on rhythm parameters (see Figure 1).

CIRCADIAN THERAPY

A drug's effectiveness, as well as the nature and extent of its side effects, may vary with the body's circadian rhythms. Evidence suggests radiotherapy and anticancer drugs are more damaging to malignant cells, and less harmful to healthy cells, depending on when treatment is provided.[7]

Since all cell proliferation is rhythmic, the optimal time to treat a cancer is when malignant and healthy cell growth and division are out of phase. This is especially important in those tissues most vulnerable to the toxic effects of chemotherapy, e.g., bone marrow and gastrointestinal mucosa cells.[7] Some antineoplastic drugs, such as cisplatin, are equally toxic to healthy and malignant cells. A major factor in gauging the best time to administer a drug is the daily rhythm of the physiological process by which it is eliminated from the body. Cisplatin is much less damaging if it is administered when renal function is maximal.[7]

Circadian optimization of chemotherapy has been reported to be highly successful with malignancies such as ovarian cancer and acute lymphoblastic leukemia (ALL) in children. Higher drug doses can be used and patient survival is increased.[7,8] Other widely used medications whose effectiveness (and adverse effects) appear to be linked to time of administration and the body's circadian rhythms include antihistamines, non-steriodal anti-inflammatory drugs, theophylline, antihypertensives, anticoagulants, and antiplatelet agents.[9]

Many aspects of the cardiovascular system show circadian rhythms, including heart rate, blood pressure, stroke volume, cardiac output, and peripheral resistance. Blood viscosity, hemoglobin, hematocrit, and epinephrine- or ADP-induced platelet aggregation are highest in the morning. In addition, fibrinolytic activity is markedly lower.(5) If you cut yourself, bleeding stops most quickly in the morning.

Pathophysiologic changes in cardiovascular disease show daily variation. The greatest risk of disease onset and appearance of symptoms is between 6 a.m. and noon.[9] For example, myocardial infarctions usually occur in the morning.

CIRCADIAN CELLULAR RHYTHMS

The time of day you encounter an antigenic substance influences the body's inflammatory reaction and the immune response. Because the circadian rhythms of circulating lymphocytes and granulocytes are of high amplitude, they have the potential of being diagnostically significant. In healthy adults, the total white blood cell count varies over a 24-hour period and peaks in the evening, usually between 9 p.m. and midnight.[11] Various leukocyte populations (e.g., neutrophils and lymphocytes) have rhythms out of phase with each other. Cells in the immune system show circadian change in number and function. Among the activities showing circadian variation are antibody and cytokine production, phagocytosis, and graft rejection. Alteration in the circadian rhythm of circulating lymphocytes has been reported as an early event in the course of HIV infection.[6,10,11] A low-amplitude circadian rhythm has been found for circulating RBCs.[11]

RHYTHMS IN BIOCHEMICAL PARAMETERS

Biochemical parameters affected by circadian rhythms will have different values at different times of day. At least 100 commonly measured analytes have been reported to show predictable, repetitive temporal variation.[4] Because many of these analytes have low-amplitude circadian rhythms, random collection of specimens is acceptable. If the amplitude of the circadian rhythm is small and close to the method imprecision, the phenomenon may be academically interesting but diagnostically irrelevant. For these analytes, rhythmic circadian fluctuation may take place within the conventionally established reference intervals.

There are other parameters, however, that show high-amplitude rhythms (Figure 2). The maximum rhythmic change of these analytes could be great enough to generate results that exceed the reference ranges. Normal ranges are often established by assaying specimens collected from the reference population in the morning, typically between 7 a.m. and 10 a.m. Analyte concentration in a specimen collected in the evening might exceed reference values derived from results conveniently, though usually arbitrarily, obtained in the morning, even in the absence of pathology. If measurements are taken near the crest of a rhythmically-fluctuating analyte, normal values may appear to be elevated (false positives). Abnormally low values may be perceived as normal (false negatives).

Assessments made near the trough of a rhythm are equally subject to misinterpretation. Serum phosphate shows a consistent upward trend during the day. After 6 p.m., the mean Pi concentration is about 20% higher than between 9 a.m. and 10 a.m[4,12] On the other hand, serum bilirubin concentrations have been reported to show a pronounced downward trend in the afternoon. Levels peak at night and reach their nadir in the late morning. After 6 p.m., the mean for serum bilirubin was reported to be 30% lower than the mean value in the morning and early afternoon.[12] A value that was elevated, yet still fell within the reference interval, would attract little clinical attention.[12]

The use of a rhythm-adjusted, rather than an arithmetic, mean takes into account the time-linked aspect of a physiologic variable and offers the clinician more useful information. The arithmetic mean does not represent the true average for a rhythmic function if data collection is not distributed equally over time and does not cover several repetitions of the basic cycle.

There are additional analytes for which specimen collection time is clinically significant. Neopterin is a metabolite derived from guanosine triphosphate. It is released by human macrophages following their stimulation by T-cell-secreted gamma interferon. Assessment of urinary levels of neopterin has been useful in the follow-up of such diseases of cellular immunity as cancer, AIDS, Crohn's disease, and sarcoidosis, as well as in monitoring allograft recipients.[13] Data show a urinary neopterin circadian rhythm of large amplitude (51%), with a peak at 6:30 a.m. the morning.[13] Because of the large time-of-day variation in that analyte, it is especially important daily level assessment, as might arise in monitoring graft rejection, always be performed on a specimen collected at the same time each day. In clinically healthy adult subjects, circadian variations of several enzymes used to detect early renal disease have been reported.[14] Circadian urinary excretion of some enzymes have been reported in healthy children between the ages of 2 and 11.[14] Large biological variations have been noted in urinary Pi, magnesium, other electrolytes, and catecholamines. Serum phosphate concentrations show a less pronounced, but still significant, periodicity.[15]

Throughout the day, from late morning (about 11 a.m.) until well past midnight (around 3 a.m.), the concentration of Pi in plasma, the excretion of Pi, and the tubular threshold of Pi resorption (TMP/GFR) increase.[16] Circadian variations have been reported by researchers in serum concentrations of clinically important lipids.[17]

ENDOCRINE FUNCTION OVER TIME

Clinical consideration of rhythmic fluctuations is routinely limited to evaluation of adrenal function. There is a circadian rhythm in the release and action of the glucocorticoids. The serum concentration of cortisol is highest shortly before, or just after, awakening, as shown in Figure 2. By afternoon, cortisol levels are usually one-half the morning values. After midnight, in the very early hours of the morning, cortisol is at its lowest. Absence of this morning/evening difference is characteristic of Cushing's disease.[9] Reference values for serum cortisol are 8 to 20 [Mu]g/dL in the morning and less than 5 [Mu]g/dL in the early evening.[18]

The immune system and the inflammatory response show a parallel fluctuation. Asthma has been characterized as probably the most circadian of all diseases. In patients with asthma, there is a day-night variation in airway function, with increased responsiveness to bronchial constrictors at night. The circadian fluctuations in cortisol contribute to the existing problem of chronically inflamed airways and reduced air flow at night, since cortisol exerts an anti-inflamatory effect. Most asthma attacks occur during the early morning hours, when serum cortisol is low and inflammatory activity is highest.[19]

Endocrine-function tests are often time dependent. A classic example is the single-dose dexamethasone suppression test used in screening patients for Cushing's syndrome. Dexamethasone, a potent glucocorticoid, is given orally to the patient at 11 p.m.; plasma cortisol is measured the following morning at 8 a.m. In normal individuals, the steroid suppresses plasma cortisol to levels less than 5 [mu]g/dL. A diagnosis of Cushing's syndrome is made if the plasma cortisol concentration fails to show a circadian rhythm and is not suppressed by dexamethasone.[9,20]

Pituitary gland hormones, as well as circulating levels of hormones from peripheral endocrine glands, vary in response to a number of factors, including time of day.[20] For example, thyrotropin (TSH) peaks near midnight in healthy adults. A TSH determination on a single specimen drawn in the evening might yield a result suggesting adequate functioning of the thyroid. Dysfunction could be missed or its diagnosis delayed until additional testing was conducted if adequacy of hormonal function was judged on the basis of the results of a single sample.

Although rhythmic changes in hormonal secretions are well documented, traditional, fixed hormonal reference ranges are still in use. Using static end points to define a "normal reference interval" has diagnostic implications, since it does not accommodate temporal variations in hormone concentration.[20] Using time-qualified (rather than arithmetic) means and establishing dynamic endpoints for reference intervals should yield more clinically meaningful information for screening, diagnosis, and treatment.(21)

SETTING YOUR BODY'S CLOCK

Melatonin, a hormone that has been described as "the chemical expression of darkness," is secreted by a pea-sized organ called the pineal gland, located in the center of the brain. The serum melatonin level usually begins to rise between 8 p.m. and 10 p.m. and peaks at about 2 a.m. to 3 a.m. By 8 in the morning, the serum melatonin level has essentially declined to its low daytime concentration.[20]

Exposure to bright light at night inhibits melatonin synthesis. The serum melatonin levels of personnel working in the well-lit lab environment will not reach the "normal" middle-of-the-night peak. A person responding to the desynchrony of short-term circadian phase-shift change is usually sleepy, tires easily, and is intermittently inattentive on the job. At home, the individual is likely to complain of bouts of insomnia and episodes of gastrointestinal disturbances. Because of its effectiveness as a sleep-inducing agent, melatonin has been advocated as a safe, inexpensive way to alleviate transient disorders of the circadian sleep-wake cycle such as jet lag or short-term shift work changes.

IMPROVED PATIENT OUTCOMES

In this era of managed care, clinicians increasingly rely on the expertise of the laboratorian to determine the best utilization of tests. In any medical situation, as the number of tests performed decreases it becomes increasingly important the limitations of those tests conducted are clearly understood. The knowledgeable laboratorian can estimate the impact of rhythmic variation on tests obtained in the emergency room laboratory or at the point of care, where testing is not restricted to standard collection times. Chronometric information can provide the basis for choosing the best analyte from among several with similar diagnostic merit. For those analytes with high-amplitude rhythms, establishing and using time-qualified reference intervals for diagnostic purposes will minimize the likelihood of a single result, such as a Stat test measurement, being misinterpreted.[17]

Morbidity and mortality statistics reveal circannual periodicity in the incidence of cardiovascular and respiratory diseases as well as for cancer and suicide.[5] The annual assessment of individual risk for certain diseases, included among them depression, breast cancer, and cardiovascular disease, makes it important for laboratorians and clinicians to be aware of particular analytes that exhibit significant yearly variation. Preliminary findings suggest both circaseptan and circannual rhythms in plasma fibrinogen, in addition to circadian rhythmicity.[23] Highest levels were found in the morning, over the weekend (Saturday to Monday), and in late spring (May and June). The prediction and treatment of coronary events could be improved by the recognition of weekly and annual patterns. Circannual variation in plasma prolactin has been correlated with breast cancer risk in studies on American and Japanese women.[5]

Plasma activity of the enzyme prolyl endopeptidase (PEP) shows significant annual variation. One pathologic condition characterized by lowered PEP serum activity is major depression. Because the high-amplitude rhythm detected in plasma prolyl endopeptidase may have diagnostic consequences, researchers have suggested population-based reference ranges be adjusted for time.[24]

While it may not be reasonable to suggest every laboratory reassess the reference ranges of all its currently tested analytes in order to find evidence of biorhythmicity, it is possible to take a first step. In the evaluation and introduction of new test procedures, there is an excellent opportunity to look for evidence of rhythms that might have diagnostic implications.[25]

FUTURE DIRECTIONS

Many opportunities exist to use the concepts of chronobiology to improve health and health care delivery. Being unaware of analyte circadian changes means taking a chance a patient will be misdiagnosed. For those analytes with strong circadian trends, advanced testing and monitoring coupled with complex, sophisticated data processing may help the lab:

* Set time-qualified reference intervals

* Set rhythm-qualified upper limits of normality

* Correct for time of specimen collection.

In the future, it should be possible to access national and international databases, derived from comparable peer populations, for this information.

A fall in amplitude or a flattening of circadian rhythms may accompany aging. Phase shifts, phase drifts, and/or loss of the body's ability to retain synchronization of biorhythms may occur among older adults. Longitudinal studies must examine the persistence or change of circadian rhythms among the elderly.[26] New public interest and media attention are focused on melatonin because of recent suggestions that, as an antioxidant, it possesses anti-aging capabilities.

We need more information on "chronorisk." The continuous monitoring of biofunctions, now possible with telemetry, indwelling devices, and noninvasive sensors and computers using chronometric software to identify temporal structure in seemingly irregular data, may provide early warning to clinicians of incipient problems in high-risk patients. In addition, chronoepidemiologists are investigating and documenting temporal patterns of selected diseases from studies of large groups of individuals.

Too often the determining factor for specimen collection or medication administration is lab or medical staff convenience. As labs consider how best to survive and thrive under managed care, selecting the best, most cost-effective test to facilitate diagnosis is essential. The benefits derived from understanding the degree and period of recurring, rhythmic changes in an analyte, in health and disease, should not be clinically or economically underestimated.[27]

References

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[15.] Halberg F, Cornelissen G. Broadly pertinent chronobiology methods quantify phosphate dynamics (chronome) in blood and urine. Clin Chem. 1992; 38:329-333.

[16.] Kemp GJ, Blumsohn A, Morris BW. Circadian changes in plasma phosphate concentration, urinary phosphate excretion, and cellular phosphate shifts. Clin Chem. 1992; 38:400-402.

[17.] Rivera-Coll A, Fuentes-Arderiu X, Diez-Noguera A. Circadian rhythmic variations in serum concentrations of clinically important lipids. Clin Chem. 1994; 40:1549-1553.

[18.] Bishop ML, Duben-Engelkirk JL, Fody EP, eds. Clinical Chemistry: Principles, Procedures, Correlations. 2nd ed. Philadelphia: Lippincott; 1992.

[19.] Hrushesky WTM. Timing is everything. The Sciences. 1994; 34(4): 32-37.

[20.] Blask DE. Chronobiology and the endocrine system. Lab Med. 1994;25:372-375.

[21.] Haus E, Touitou Y. Chronobiology in laboratory medicine. In: Touitou Y, Haus E, eds. Biologic Rhythms in Clinical Laboratory Medicine. 1992; Berlin: Springer-Verlag; 673-708.

[22.] Reiter R. Melatonin: Multifaceted messenger to the masses. Lab Med. 1994; 25:438-443.

[23.] Kanabrocki EL, Sothern RB, Bremner F, et al. Weekly and yearly rhythms in plasma fibrinogen in hospitalized male military veterans. Am J Cardiol 1995; 76: 628-631.

[24.] Maes M, Scharpe S, De Meester I, et al. Components of biologic variation in prolyl endopeptidase and dipeptidyl-peptidase IV activity in plasma of healthy subjects. Clin Chem. 1994; 40(9): 1686-1691.

[25.] Fraser CG. Data on biological variation: Essential prerequisites for introducing new procedures. Clin Chem. 1994; 40(9):1671-1673.

[26.] Kanabrocki EL, Sothern RB, Scheving LE, Halberg F, et al. Ten-year-replicated circadian profiles for 36 physiological, serological, and urinary variables in healthy men. Chronobiol Int. 1988; 5: 237-84.

[27.] Halberg F, Cornelissen G, Bakken E. Caregiving merged with chronobiologic outcome assessment, research, and education in health maintenance organizations. In: Hayes D, Pauly J, Reiter R, eds. Chronobiology: Its Role in Clinical Medicine, General Biology, and Agriculture. Part B. 1990; New York, NY: Wiley-Liss Inc.; 491-549.