Standardised Resting Time Prior to Blood Sampling and Diurnal Variation Associated with Risk of Patient Misclassification: Results from Selected Biochemical Components

Abstract

Background: According to current recommendations, blood samples should be taken in the morning after 15 minutes' resting time. Some components exhibit diurnal variation and in response to pressures to expand opening hours and reduce waiting time, the aims of this study were to investigate the impact of resting time prior to blood sampling and diurnal variation on biochemical components, including albumin, thyrotropin (TSH), total calcium and sodium in plasma.

Methods: All patients referred to an outpatient clinic for blood sampling were included in the period Nov 2011 until June 2014 (opening hours: 7am-3pm). Each patient's arrival time and time of blood sampling were registered. The impact of resting time and the time of day for all components was analysed using simple linear regression. The "maximum allowable bias" was used as quality indicator for the change in reference interval.

Results: Significant diurnal variation was found for albumin (n = 15,544; p<2×10-16), TSH (n = 20,019; p<2×10-16), calcium (n = 13,588; p = 2.8×10-12) and sodium (n = 51,917; p<2×10-16). Further significant influence for resting time was found for albumin (p = 2.6×10-4), TSH (p = 0.004), calcium (p = 8.9×10-7) and sodium (p = 8.7×10-16). Only TSH and albumin were clinically significantly influenced by diurnal variation. Resting time had no clinically significant effect.

Conclusions: We found no need for resting 15 minutes prior to blood sampling. However, diurnal variation was found to have a significant and considerable impact on TSH and, to a minor degree, albumin. This has to be taken into account to ensure that reference intervals provided by the laboratory are valid on a 24-hour basis.

Fig 1. Diurnal variation.

The figures illustrate the diurnal variation of A) albumin, B) thyrotropin, C) calcium and D) sodium divided into one-hour intervals. The values above the columns show the number of patients in each time interval. The shadowed background field area indicates the upper and lower limit defined by the maximum allowable bias.

Fig 2. Misclassification of patients relative to the reference interval.

Dividing the patients in three subgroups based on the reference interval produces “Below”, “Ref. interval” and “Above”. The table indicates the number of patients in each subgroup. The graph shows the same numbers in % for the subgroups “Below” and “Above”. The values in the “Misclassification” box are the number of patients misclassified at 2–3pm relative to 7–8am.

Fig 3. Total impact of resting time and diurnal variation on the biochemical compounds.

The figure shows the change in mean value (in %) relative to the mean under the reference conditions (7–9am, resting time 15–30 minutes) for the four components A) albumin, B) thyrotropin, C) calcium and D) sodium. The shadowed background field indicates the limits for the MAB (1/3×SD_pop). The values above the columns are the number of patients in each interval.