Seeing Red: Hydroxocobalamin and Result Interference

Published:March 16, 2023DOI:
      To the Editor:
      Intravenous high dose hydroxocobalamin (CyanoKit®, BTG Pharmaceuticals, London, United Kingdom) is indicated for the treatment of cyanide toxicity.1 In the last 10 years, the nitric oxide scavenging properties of hydroxocobalamin have brought it to the forefront for use in the treatment of vasoplegia.1–3 Despite a lack of randomized evidence, many retrospective studies have demonstrated its value in this context, and hydroxocobalamin has a number of characteristics favorable to critically ill patients with vasoplegia including a lack of serotonergic activity or evidence of contributing to a hypercoagulable state.1,4,5 Sequela of hydroxocobalamin are varied and include chromaturia, rash, diffuse erythema, and elevated cobalt levels.1,6 Carlsson et al describe hydroxocobalamin absorption spectrum peaks of 274 nm (ultraviolet light) and 351 nm (ultraviolet light but nearly violet which is 380 nm) as well as a broad, smaller absorption range with slight peaks at 500 nm (cyan) and 526 nm (cyan to green).7–9 Importantly, the lack of absorption above 600 nm is what confers the red color of the hydroxocobalamin solution as well as chromaturia, and the appearance of diffuse skin erythema.1 When given intravenously in high doses, hydroxocobalamin remains in the body for weeks after administration.10 With its increasing use in the treatment of vasoplegia, hydroxocobalamin's interference with colorimetric laboratory assays as well as monitors that use spectrophotometry or colorimetry is a growing concern.1,11,12

      Laboratory Testing

      The laboratory interference of hydroxocobalamin varies not only by study type, but also the individual laboratory of interest and by the analyzer.1,7 There are a variety of laboratory analyzers for the many routine tests, each using their own, often proprietary technology and methodology. For example, glucose, a commonly analyzed molecule, has negligible absorption wavelengths less than 1100nm, and therefore would not be expected to be impacted by hydroxocobalamin.13 But, a seemingly straightforward test, like glucometry, may be performed using several different methodologies, with some being impacted by hydroxocobalamin's light absorption, while others are not.
      Based on the published literature, the duration of interference for blood tests does not exceed 48 hours, but urinalysis and dialysis interference can last for 8 or more days.1,12 Plasma collected from patients who have received hydroxocobalamin may appear as a grossly hemolyzed sample, though may not necessarily flag for hemolysis when analyzed.14 Results that tend to be unaffected by high dose intravenous hydroxocobalamin administration include calcium, sodium, potassium, chloride, and urea as well as hematocrit, platelets, and eosinophils.1,14 Laboratory results that are thought to be artificially increased by at least 10% after intravenous hydroxocobalamin administration include PaO2, creatinine, albumin, glucose, hemoglobin, and alkaline phosphatase.15 Laboratory results thought to be artificially decreased by at least 10% include alanine aminotransferase and amylase.1 Interestingly, numerous laboratory investigations have unpredictable interference including phosphate, aspartate aminotransferase, creatine kinase, creatine kinase-myocardial band, lactate dehydrogenase, activated partial thromboplastin time (aPTT), prothrombin time (PT) and the international normalized ratio (INR).1 We suspect that some of this unpredictability may not be completely attributable to the molecules being tested, but rather, potentially differing analyzers using different methodologies. When clinicians report on cases of new or unique interference, it is therefore valuable to know what specific analyzer was used for laboratory analysis. In a recent report, Cagle et al describe hydroxocobalamin's significant interference with the chromogenic anti-Xa assay in a patient supported with extracorporeal membrane oxygenation (ECMO).15 Though in this case, anti-Xa was undetectable in the setting of heparinization, at the moment, there is insufficient evidence to comment whether over or underestimation occurs routinely with this test. Authors have examined the use of the hemolysis index to indirectly approximate the concentration of hydroxocobalamin.16 From this, the authors were able to approximate the degree of error and provide a corrective linear equation for creatinine and lactate.16 Interestingly, a repeat study with the use of increasing concentrations of hydroxocobalamin did not result in an elevated hemolysis index but did result in the analyzer to flag the specimen (meaning the specimen sample either is out of the range of the assay or that there is an issue with the specimen sample).14 Further research on this subject matter is still needed and it is likely that different analyzers will yield different results.
      In the case of urinalysis, chromaturia can appear similar to hematuria. Notably, hydroxocobalamin can affect urine color for 28 days or more, and urinalysis results (including nitrites, pH, and glucose) can be erroneous for 8 days.1,12

      Point of Care Testing

      In the perioperative period, there are a variety of point-of-care (POC) tests that are routinely performed, and in the setting of cardiac surgery, and post-cardiac surgery intensive care, it is important to consider whether these may be impacted by hydroxocobalamin when making clinical decisions. Activated clotting time (ACT), thromboelastography (TEG, Haemonetics, Boston MA, USA) or rotational thromboelastometry (ROTEM, Werfen, Barcelonna, Spain) are all frequently encountered POC tests of coagulation in cardiac surgery. ACT is usually measured mechanically, electromechanically, via plasma conductivity (amperometry) or with an optical sensor. If ACT is measured electromechanically (Medtronic ACT Plus, TEG, ROTEM), or amperometrically (I-STAT ACT, Abbott, Chicago IL, USA), then there should be no artificial interference in the measurement by hydroxocobalamin.17 Though ROTEM does use an optical sensor, this sensor does not measure the blood sample directly, and therefore should not be impacted by hydroxocobalamin. However, it is conceivable that hydroxocobalamin could interfere with the optical sensor method of measuring activated clotting time (hemochron Signature Elite, Werfern, Barcelona, Spain), though no reports of this phenomenon exist.18 A recent report of possible chromogenic factor X interference by intravenous hydroxocobalamin administration provided some evidence that TEG values likely accurately reflected anticoagulation status.15
      Unlike some laboratory analyses, I-STAT for measurement of other laboratory values, such as blood chemistry, blood gas, hemoglobin and hematocrit should not be impacted by hydroxocobalamin. As previously mentioned, though glucose should not be meaningfully impacted by hydroxocobalamin, there is emerging evidence that it may significantly impact POC glucometry.19 Unfortunately, there are numerous other commercially available blood gas analyzers on the market worldwide, and as a result, clinicians who administer hydroxocobalamin need to refer to the user manual of their institutions POC device to determine if it uses an optical sensor for measurement and may be subject to interference by hydroxocobalamin.


      Beyond laboratory assay interference, monitors can also be affected by high plasma concentrations of hydroxocobalamin. Pulse oximetry, co-oximetry, dialysis machines, and other continuous blood monitors such as those used for ECMO circuit monitoring come to mind as possible measurements that could be affected by the administration of hydroxocobalamin.

      Co-Oximetry and Pulse Oximetry

      Non-invasive co-oximetry is widely used and utilizes, among others, 660 nm and 940 nm wavelengths (these two wavelengths are also used by pulse oximetry) to estimate oxyhemoglobin saturation. A more advanced form of pulse oximetry, known as co-oximetry, can detect other forms of hemoglobin and may be altered by high blood levels of hydroxocobalamin. For example, a report of two patients suggests that patients that received high dose intravenous hydroxocobalamin may have artificial low carbon monoxide levels as measured by oximetry.11 In a study by Cashin et. al, seven healthy volunteers that were administered 5 grams of intravenous hydroxocobalamin were found to have no difference in estimated oxyhemoglobin saturation, carboxyhemoglobin saturation, or non-invasively measured hemoglobin levels; measured methemoglobin levels were slightly altered.20 Though the sample size is small, this work does suggest that co-oximetry may provide acceptable measured hemoglobin oxygen saturations in patients treated with hydroxocobalamin.

      Blood Leak Alarms in Renal Replacement Therapy

      During renal replacement therapy, dialysate bags are monitored for blood to prevent patient harm.21 These sensors are typically either detect blood leakage via optical sensors or with colorimetry.21 Hydroxocobalamin is small enough to pass through the membrane and accumulate in the dialysate bag. Due to the method of blood leak detection in many renal replacement devices, the blood leak alarm can be falsely triggered by high levels of hydroxocobalamin and cause the device to alarm and, as a safety mechanism, cease to function.12,21 Individual cases and case series have been reported of false blood leak alarm activation during intermittent hemodialysis in patients treated with high dose intravenous hydroxocobalamin.12,21 Hydroxocobalamin can pigment dialysate bags for some time (in one report, 9 days) and as a result, patients may require continuous renal replacement therapy until blood levels are low enough to not trigger the blood leak alarms on intermittent dialysis devices.12 Though reports exist of providers deactivating the blood leak alarm, this does carry the risk of overlooking a true blood leak.12 Given that patients with vasoplegia are generally critically ill and hemodynamically compromised, if a patient with vasoplegia requires renal replacement therapy it seems wise to proceed with continuous renal replacement therapy (as opposed to intermittent) for its more stable hemodynamic profile.

      ECMO Sensors

      For ECMO circuits, multiple monitors are necessary for patient safety. Blood flow and bubble detector sensors (which can be placed directly on ECMO tubing) utilize ultrasound waves to detect information and thus would not be affected by hydroxocobalamin administration.22 In-line measurements of hemoglobin oxygen saturation are also available, but some devices use 510 nm and 600 nm wavelengths which could be interfered with by hydroxocobalamin administration (with a risk of overestimating true oxygen saturation given the absorption spectrum of hydroxocobalamin).23 Abnormal values should be correlated with arterial blood gas analysis and the clinical picture.


      Providers that may be routinely using hydroxocobalamin should consider meeting with stakeholders, including the directors of laboratories, nephrologists, and ECMO specialists to discuss the likelihood of erroneous results after the administration of this medication. Given the wide variability in analyzer methodologies, it may be wise to run institutional studies on samples with hydroxocobalamin for quality control of analyzers as well. It may also be wise to consider implementing institutional protocols for the interpretation laboratory results in this patient population, such as flags in the electronic medical record that point out the possibility of error for 24 to 48 hours after hydroxocobalamin administration (and indeed for up to 8 days in the case of certain urinalysis results).
      At the individual level, if a patient with vasoplegia is likely to receive treatment with hydroxocobalamin, we recommend sending off baseline blood tests prior to administration. Additionally, all providers should be aware of the administration of hydroxocobalamin prior to its initiation so that abnormal laboratory values, urine color, chest tube output color, and blood color can be anticipated and not cause alarm or unilateral changes in management. Important tests to obtain prior to hydroxocobalamin include arterial blood gas analysis, complete blood count with differential, basic metabolic panel, calcium, magnesium, phosphorus, liver function tests, aPTT, PT, INR, fibrinogen, amylase, lipase, and chromogenic anti-Xa. This offers providers a valuable baseline reference prior to medication administration. One option to ensure consistency is to include a set of baseline laboratory tests in an order set for hydroxocobalamin. In the following 24 to 48 hours, laboratory values should continue to be drawn per institutional protocol but there should be a high index of suspicion for erroneous results. Education should be provided to downstream staff, including intensive care nurses, of the potential impacts of hydroxocobalamin on various laboratory results and monitors.


      Hydroxocobalamin will likely continue to play an ever more prominent role in the management of vasoplegia. With the vast number of laboratory assays available, it is almost certain that as time goes on, additional reports of erroneous results will continue. It is also very important that providers be aware and plan accordingly for colorimetric and photometric sensors that could display altered measurements in the setting of hydroxocobalamin administration. As always, when seemingly spurious results that confuse the overall picture are reported, we suggest that clinical judgement with the aid of experienced providers is the best way to decide the course of action. The phenomenon of interference with laboratory values, certain monitors, and dialysis machine blood leak alarms is a key concept for anesthesiologists, intensivists, surgeons, cardiologists, nephrologists, nurses, ECMO specialists, perfusionist and other providers to understand and anticipate.

      Declaration of Competing Interest

      We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.


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