Vitamin D assessment in perioperative medicine and critical care

Low vitamin D status was reported to be significantly associated with a poor outcome in critically ill patients in several studies over the last decade [1‐3]. These findings led to the performance of one large randomized clinical trial of high-dose vitamin D supplementation, albeit an overall negative trial, which sparked further scientific and clinical interest because a mortality benefit was found in a prespecified subgroup of patients deemed to be severely vitamin D deficient [4].

Since poor vitamin D status may contribute to excess mortality in critical illnesses, vitamin D treatment seems to be feasible, safe and possibly advantageous. As vitamin D deficiency is also highly prevalent in this setting, high-dose vitamin D treatment [5] may become an adjunctive therapy in critical care in the future; however, patient selection is important and some data suggest that the definition of deficiency may be different in this patient population [6].

Some studies in both critically ill and non-critically ill patients have raised the possibility that vitamin D status is inversely associated with inflammation and that levels may decrease rapidly during acute inflammation as well as fluid loading [7, 8]. Low vitamin D levels have been suggested to be reflective of the severity of illness in critically ill patients rather than being an independent influencing factor on outcome [9, 10]. The use of 25-hydroxyvitamin D [25(OH)D] may be unreliable in critical illness, since both 25(OH)D and serum vitamin D binding protein (VDBP) have been shown to significantly decrease during inflammation [8, 9]. Additionally, there is ongoing debate about which vitamin D metabolite is best suited to represent the vitamin D status in critically ill patients and which methods should be employed [11]. Measurement of serum 25(OH)D by LC-MS/MS has been found to be superior to the use of immunoassays in non-critically ill patients who received a fluid bolus of crystalloid solution [12].

When turnaround times should be minimal the use of automated readily available methods of 25(OH)D testing is certainly preferable to the time-consuming method of LC-MS/MS in clinical routine [13, 14]. International standardization of current 25(OH)D immunoassays has reduced previously reported interlaboratory variation [15, 16]; however, differences seem to be more prominent in specific populations, such as hemodialysis patients, critically ill individuals or pregnant women [17, 18]. Regular LC-MS methods may also detect the biologically inactive C3-epimer which may significantly contribute to 25(OH)D concentrations and can therefore lead to overestimation of 25(OH)D levels [19, 20]. The percentage of this inactive epimer seems to be higher than reported in other populations but its role in critical care is currently unknown.

A total of 26 patients providing 130 samples for analysis were included in this analysis. These patients were predominately male (n = 19, 73%), all of Caucasian ethnicity, with a median age of 67 years (range 60–76 years). Patients were treated in the ICU for a median of 3 days (2–6 days) and remained hospitalized for a median of 14 days (13–17 days). Both in the ICU and in the hospital there were no fatalities (n = 0, 0%) in this cohort (Table 1).

Patients were treated according to local protocol based on clinical needs as assessed by the treating physicians. Parameters of physical status, relevant routine laboratory measurements and fluid balance are shown in Table 2.

In this study it was demonstrated that changes during the perioperative phase and critical illness significantly altered both the findings of a commonly used chemiluminescence assay for vitamin D measurements and mass spectrometry. Acute fluid loading by CPB leads to a reduction in 25(OH)D levels of about one fifth of baseline values on an individual patient level. This is somewhat lower than in a previous small study in patients undergoing major cardiac surgery [7] and other studies in non-critically ill patients [8], which suggested a reduction up to 40% of baseline levels. While this reduction in serum 25(OH)D levels is physiologically plausible following a single significant volume expansion (median 3.7l, IQR 3.1–4.1l) by CPB, interindividual variation in response was surprisingly high. Inference of premorbid vitamin D status from values measured later on therefore seems improper, especially in critically ill patients, whose volume status may be influenced by far more sources than one fluid bolus alone. This is confirmed by limited linear correlations between fluid balance as well as markers of inflammation and relative changes in 25(OH)D levels in this study. These alterations in vitamin D status in states of critical illness may contribute to overdiagnosis and potentially overtreatment later on. While supplementation of cholecalciferol during intensive care can be considered standard of care using doses recommended for the overall population (600–4000 IU daily), high bolus doses have been shown to be necessary in critical illness if correction of vitamin D deficiency is the goal [22]. In the only large randomized clinical trial investigating this intervention published to date, no significant benefit was found regarding the primary endpoint (length of stay) or overall mortality [4]. There was, however, a statistically significant and clinically potentially highly relevant reduction in mortality in severe vitamin D deficiency (n = 200/42% of the total population, 25(OH)D levels < 12 ng/ml).

More specific patient selection is a possible explanation for this subgroup finding. Selecting patients that benefit most from interventions both in clinical trials and routine practise is considered ever more important in times of “precision medicine” [6, 23]. This alternative cut-off for diagnosis of vitamin D deficiency was therefore tested in critically ill patients after fluid loading in this study. As could be expected from a narrower definition of the condition in question, the lower cut-off was indeed more specific for the diagnosis of vitamin D deficiency when preoperative 25(OH)D levels lower than 20 ng/ml measured by LC-MS/MS were used as the gold standard for comparison. Conversely, the rate of false negative findings would also be higher.

Readily available, economically reasonable and sufficiently specific tests are preferable both in everyday clinical practice and for conducting clinical trials to maximize the potential benefit of an intervention yet to be proven, such as high-dose vitamin D supplementation in critically ill patients. The reliability of a commercially available chemiluminescence assay (ECLIA) compared to mass spectrometry, the gold standard in this field was therefore assessed. Although the detection of the biologically inactive C3 epimer was claimed to lead to overestimation of vitamin D levels by LC-MS/MS, the findings refute this notion as no epimers were detected by LC-MS/MS, further corroborating its standing as the gold standard method.

Diagnosis of vitamin D deficiency in the status of fluid loading and inflammation, such as the perioperative period and critical illness, can be complicated by hemodilution. Inference of baseline values from downstream measurements is not reliably possible. Stricter definitions, such as serum 25(OH)D levels lower than 12 ng/ml may therefore be used to diagnose vitamin D deficiency with a low false positive rate. Measurements by commonly available chemiluminescence assays may deviate noticeably from the mass spectrometry gold standard; however, they are acceptably reliable in the acute setting when vitamin D deficiency has to be diagnosed.