Diurnal Variation

Our business is finding the unexpected. As a bench tech, you may look at hundreds of test results each day, many of which are routinely normal. More and more these kinds of results are verified by software algorithms. This leaves the abnormal, critical, or unexpected result to verify. Identifying diurnal variation can help you decide if an unexpected result needs to be investigated further.

Sources of Variation
Dr. Callum Fraser identifies three factors that can vary test results over time, explaining why analyzing a serial samples can yield different numbers:1

? Pre-analytical – patient factors that influence the specimen at time of collection, including posture, the amount of time the tourniquet is applied, etc.

? Analytical error – including systematic and random measurement errors.

? Biological variation around a homeostatic set point – also called within-subject or intra-individual biological variation.

On the bench it’s usually all about analytical errors that affect result accuracy. Many of our quality processes are designed to identify these kinds of errors. Systematic errors are consistent and generally caused by a problem with an instrument or how it is used. Linear methods, such as those on a chemistry instrument, demonstrate a measurement bias if the offset or slope of a calibration is incorrect. Random errors, by contrast, are inconsistent and do not demonstrate a bias.2, 3

Given the above, the challenge for a technologist is identifying when variation equals error. Systematic can be easier to detect than random error, because they tend to affect all results. For example, a problem with temperature calibration can change enzyme test results; this bias can be found during routine maintenance or quality control checks. But gradual system deterioration e.g. probe debris causes a more subtle bias.

That leaves random error that can be confused with natural biological variation. Recognizing the latter will help you identify the former.

Diurnal Variation
Our bodies are subject to various biological rhythms throughout the day and from day to day that affect laboratory test results. Two common examples are diurnal and circadian rhythms. Diurnal variation refers to a daily cycle or during the day e.g. the opposite of nocturnal. Circadian rhythms occur in approximately 24-hour periods, often reset by environmental cues. (Although, interestingly, our internal clock runs on a 25-hour cycle.) While the latter is a subset of the former, both terms refer to cyclic changes; almost all diurnal variation is circadian.4

Body temperature is one example. During active daylight hours as we consume calories and use muscles our temperature peaks around four p.m. At night we are in a “steady state,” by contrast, with a trough temperature around four a.m. Factors that influence these rhythms may be endogenous (internal cues driven by hormones or our body clock located in the hypothalamus) or exogenous (external cues such as sunlight). Generally, these factors complement each other but are disrupted by shift work and other changes.5

Your staff may observe patients complaining of feeling cold under covers if a specimen has to be collected in the middle of the night, yet these same patients can seem comfortable sitting in a chair the following afternoon. Circadian rhythm is one possible explanation.

Laboratory Diurnal Variation
Here are a few common examples of laboratory value diurnal variation:

? Thyroid Stimulating Hormone (TSH) – TSH values can vary by as much as fifty percent throughout the day, rising several hours before sleep, peaking between 2300 and 0600, and reaching trough by the afternoon.6

? Calcium – Calcium peaks in the evening and decreases thereafter to reach lowest levels between 0200 and 0400, varying by as much as 0.5 mg/dL according to one study.7

? White blood cells – One study of clozapine treatment concluded that leukocyte counts show a significant diurnal variation of nearly two thousand per microliter between morning and afternoon. Your patients may exhibit a “pseudoneutropenia” in the early morning.8

? Iron – Morning iron levels are higher than afternoon or evening levels, although this variation is inconsistent between individuals. Morning and afternoon measurements may be useful.9, 10

Diurnal variation in other endogenous (blood pressure, etc.) and exogenous (fluid intake, etc.) factors may affect other test results, exacerbated by clinical conditions. For example, diurnal variation in glomerular filtration rate (GFR) and albuminuria has been demonstrated in patients with diabetic neuropathy (a 20% and 50% rise, respectively, from night- to daytime).11 Thus, you might expect to see diurnal variation in normal patients and unexpected diurnal variation in abnormal patients.

Finally, variation can be artificial depending on treatment received. Patients receiving fluids, diuretics, or blood products can exhibit sudden changes in laboratory values. This variation can often be the most difficult to judge, but a quick consult with the patient’s nurse or caregiver can often resolve the issue of expected vs. unexpected variation.

Your Quality System

In bench practice concepts like diurnal variation can be challenging to apply. The stereotype that the laboratory treats specimens is true to an extent: techs often don’t see the patient or have access to diagnostic information to make a good judgment. Patient condition and treatment variables present a picture understood and managed by the physician. But if our job is finding the unexpected value, knowing that diurnal variation is expected should be integrated into a laboratory quality system.

Most laboratory information systems can perform delta checking that flags differences between current and previous test values. A delta check policy, for example, can describe how a bench technologist looks for preanalytical error, sampling errors, or expected patterns e.g. a run of negative delta checks on one analyte. Diurnal variation can be included in these checks as a possible explanation to consider once other factors are ruled out.

If preanalytical, analytical, and diurnal causes of variation are ruled out, it never hurts to call the patient’s nurse or provider to get more information when a result is still unexpected. Making that call and asking questions to help the rest of the team promotes your laboratory as a quality resource. It also leads to better patient care.

Scott Warner is laboratory manager, Penobscot Valley Hospital, Lincoln, Maine.


1. Fraser C. Biologic Variation: Principles and Practice – Westgard QC. Westgardcom. Available at: https://www.westgard.com/biologic-variation-principles-and-practice.htm. Accessed July 26, 2014.

2. E-education.psu.edu. 5. Systematic vs. Random Errors | The Nature of Geographic Information. Available at: https://www.e-education.psu.edu/natureofgeoinfo/c5_p5.html. Accessed July 26, 2014.

3. Physics.umd.edu. Random vs. Systematic Error. Available at: http://www.physics.umd.edu/courses/Phys276/Hill/Information/Notes/ErrorAnalysis.html. Accessed July 26, 2014.

4. Vitaterna M, Takahashi J, Turek F. Overview of Circadian Rhythms.National Institute on Alcohol Abuse and Alcoholism. Available at: http://pubs.niaaa.nih.gov/publications/arh25-2/85-93.htm. Accessed August 3, 2014.

5. Acep.org. Circadian Rhythms and Shift Work. 2003. Available at: http://www.acep.org/Clinical—Practice-Management/Circadian-Rhythms-and-Shift-Work/. Accessed August 3, 2014.

6. Fisher MD D, Nelson MD J. Application of TSH and Free Thyroxine Measurements to Thyroid Diagnosis. Questdiagnosticscom. 2012. Available at: http://www.questdiagnostics.com/testcenter/testguide.action?dc=WP_AppTSH. Accessed August 4, 2014.

7. Even L, Badar T, Hochberg Z. Nocturnal calcium, phosphorus, and parathyroid hormone in the diagnosis of concealed and subclinical hypoparathyroidism. European Society of Endocrinology. 2007. Available at: http://www.eje-online.org/content/156/1/113.full.pdf. Accessed August 4, 2014.

8. Ahokas A, Elonen E. Circadian rhythm of white blood cells during clozapine treatment (Abstracs) [Psychopharmacology (Berl). 1999] – PubMed – NCBI. Ncbinlmnihgov. 1999. Available at: http://www.ncbi.nlm.nih.gov/pubmed/10435399. Accessed August 5, 2014.

9. Long R, Delaney K, Siegel L. Diurnal variation of serum iron in normal individuals. Clinical Chemistry. 1978;24(5):842-842. Available at: http://www.clinchem.org/content/24/5/842.1.full.pdf+html. Accessed August 5, 2014.

10. Dale J, Burritt M, Zinsmeister A. Diurnal variation of serum iron, iron-binding capacity, transferrin saturation, and ferritin levels (Abstract) [Am J Clin Pathol. 2002] – PubMed – NCBI. Ncbinlmnihgov. 2002. Available at: http://www.ncbi.nlm.nih.gov/pubmed/12090432. Accessed August 5, 2014.

11. Hansen H, Hovind P, Jensen B, Parving H. Diurnal variations of glomerular filtration rate and albuminuria in diabetic nephropathy. Kidney International. 2002;61(1):163. doi:doi:10.1046/j.1523-1755.2002.00092.x.

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