Antibody-mediated interferences affecting cardiac troponin assays: recommendations from the IFCC Committee on Clinical Applications of Cardiac Biomarkers

The International Federation of Clinical Chemistry Committee on Clinical Applications of Cardiac Biomarkers (IFCC C-CB) provides educational documents to facilitate the interpretation and use of cardiac biomarkers in clinical laboratories and practice. Our aim is to improve the understanding of certain key analytical and clinical aspects of cardiac biomarkers and how these may interplay. Measurements of cardiac troponin (cTn) have a prominent place in the clinical work-up of patients with suspected acute coronary syndrome. It is therefore important that clinical laboratories know how to recognize and assess analytical issues. Two emerging analytical issues resulting in falsely high cTn concentrations, often several fold higher than the upper reference limit (URL), are antibody-mediated assay interference due to long-lived cTn-antibody complexes, called macrotroponin, and crosslinking antibodies that are frequently referred to as heterophilic antibodies. We provide an overview of antibody-mediated cTn assay interference and provide recommendations on how to confirm the interference and interpret the results.

Total bilirubin assay differences may cause inconsistent treatment decisions in neonatal hyperbilirubinaemia

OBJECTIVES

To assess interlaboratory variability of total serum bilirubin (TSB) results in newborns. Initiated following a clinical incident in which a neonate was transferred to a tertiary hospital for treatment of severe hyperbilirubinemia but on arrival was reclassified into a lower risk category due to a 20% difference in TSB between laboratories.

METHODS

Fresh residual plasma samples from hospital-born infants were pooled to obtain 11 samples across a range of total bilirubin concentrations. Aliquots were light-protected and measured on 7 commercial platforms at 4 accredited medical laboratories. Data from The Royal College of Pathologists of Australasia Quality Assurance Programs' (RCPAQAP) Neonatal Bilirubin program was analysed.

RESULTS

Twenty-four to 30% difference in results for individual samples, largely due to calibration differences between assays. When interpreted according to guidelines, results from different platforms would have led to different clinical interventions in some cases. RCPAQAP results showed significant within-method bias but were not shown to be commutable with patient samples.

CONCLUSIONS

There are clinically significant method-dependent differences in TSB results from neonatal samples, consistent with our clinical incident. The differences are largely due to lack of standardisation of calibrator values. This has implications for healthcare resource use and possibly for the neurodevelopment of infants. Intervention is needed at a number of levels, including clinical reporting of incidents arising from discordant results, commitment by manufacturers to ensure metrological traceability of methods with sufficiently low uncertainty in the final measurements, and availability of commutable quality assurance material to monitor assay performance, especially at the clinical decision points for neonatal jaundice.

High incidence of macrotroponin I with a high-sensitivity troponin I assay

Background:

Cardiac troponin is the preferred biomarker of myocardial injury. High-sensitivity troponin assays allow measurement of very low levels of troponin with excellent precision. After the introduction of a high-sensitivity troponin I assay the laboratory began to receive enquiries from clinicians about clinically discordant elevated troponin I results. This led to a systematic investigation and characterisation of the cause.

Methods:

Routine clinical samples were measured by the Architect High Sensitive Troponin-I (hsTnI) and the VITROS Troponin I ES assays (VitrosTnI). Results that were elevated according to the Architect but not the VITROS assay (Group 1) or results elevated by both assays but disproportionately higher on the Architect (Group 2) were re-analysed for hsTnI after re-centrifugation, multiple dilutions, incubation with heterophilic blocking reagents, polyethylene glycol (PEG) precipitation, and Protein A/G/L treatment. Sephacryl S-300 HR gel filtration chromatography (GFC) was performed on selected specimens.

Results:

A high molecular weight complex containing immunoreactive troponin I and immunoglobulin (macrotroponin I) was identified in 5% of patients with elevated hsTnI. Patients with both macrotroponin and myocardial injury had higher and longer elevation of hsTnI compared with VitrosTnI with peaks of both macrotroponin and free troponin I-C complex on GFC.

Conclusions:

Circulating macrotroponin I (macroTnI) causes elevated hsTnI results with the Architect High Sensitive Troponin-I assay with the potential to be clinically misleading. The assay involved in this investigation may not be the only assay affected by macrotroponin. It is important for laboratories and clinicians to be aware of and develop processes to identify and manage specimens with elevated results due to macrotroponin.

Genotyping FOXG1 Mutations in Patients with Clinical Evidence of the FOXG1 Syndrome

Rett syndrome is a well-defined neurodevelopmental disorder comprising characteristic clinical features of gait abnormalities, loss of purposeful hand movements, stereotypies, and autistic features. Mutations in the FOXG1 gene have been associated with a congenital variant of Rett syndrome. This is a report on the outcome of routine genetic testing to identify FOXG1 mutations in a patient population presenting with features of the FOXG1 syndrome, an entity thought to be distinct, but similar, to the congenital variant of Rett syndrome. We performed PCR and sequencing analysis of FOXG1 in MECP2-negative patients (n = 12) with phenotypic features of FOXG1 syndrome. FOXG1 MLPA analysis was also carried out. No mutations in FOXG1 were identified using this approach. We were unable to identify patients with features of the FOXG1 syndrome as having aberrant FOXG1 gene loci. Clinical notes are inherently subjective and may lack sufficient detail to reliably identify those with a syndromal spectrum. The results call into question the objectivity of outlining a complex syndrome according to clinical manifestations and highlight the need for a greater involvement of molecular diagnostic techniques in the diagnosis of Rett-like disorders.

Associations of serum vitamin D concentrations with obstetric glucose metabolism in a subset of the Hyperglycemia and Adverse Pregnancy Outcome (HAPO) study cohort

AIMS

To assess associations between maternal serum vitamin D concentration and glucose metabolism in a cohort of pregnant women living in an Australian subtropical environment.

METHODS

Cross-sectional assessment of 25-hydroxy vitamin D concentrations in 399 Hyperglycemia and Adverse Pregnancy Outcome ancillary study participants, treated at an obstetric teaching hospital in Brisbane, Australia. All patients underwent a blinded 75-g oral glucose tolerance test at 24-32 (target 28) weeks' gestation.

RESULTS

The mean (± standard deviation) fasting plasma glucose was 4.5 ± 0.4 mmol/l. Mean (± standard deviation) serum 25-hydroxy vitamin D was 132.5 ± 44.0 nmol/l. A difference of one standard deviation in maternal 25-hydroxy vitamin D was inversely related to fasting glucose (fasting glucose lower by 0.047 mmol/l, P=0.012) when assessed with multiple linear regression after adjusting for confounders. Maternal 25-hydroxy vitamin D correlated with β-cell function as estimated by the log-transformed homeostasis model assessment-β-cell function equation (r=0.131, P=0.009), but not with the homeostasis model assessment of insulin resistance.

CONCLUSIONS

An association between mid-gestational 25-hydroxy vitamin D and fasting glucose was confirmed in a largely normoglycaemic and vitamin D-replete pregnant population. The correlation between 25-hydroxy vitamin D and β-cell function suggests that vitamin D may influence glucose metabolism through this mechanism. Intervention studies are required to determine causality and the role of vitamin D replacement in deficient individuals.

Can one point-of-care glucose meter be used for all pediatric and adult hospital patients? Evaluation of three meters, including recently modified test strips

BACKGROUND

within hospitals, bedside blood glucose measurements are widely used for investigating suspected hyper- or hypoglycemia, monitoring diabetes, and adjusting glucose-lowering medication. Numerous point-of-care glucose meters are available, but for large hospitals using the same meter in all settings has practical and economic advantages. This investigation sought to identify a meter that was accurate, precise, and free from interferences, making it suitable for use across all ages and diseases.

METHODS

lithium-heparinized whole blood was analyzed, under various conditions, on the HemoCue Glucose 201 (Hemocue AB, Ängelhom, Sweden), Accu-Chek Performa (Roche Diagnostics, Basel Switzerland) (using the newly reformulated maltose-insensitive strips), and Optium (Abbott Diabetes, Alameda, CA, USA) glucose meters and compared with plasma glucose measurements on the Vitros 5,1 FS analyzer (Ortho Clinical Diagnostics, Neckargemund, Germany).

RESULTS

biases of 3.2%, -5.8%, and -8% were found with Accu-Chek, Optium, and HemoCue, respectively. Within-run imprecision was 2.5-5.8%. Between-run imprecision was 3.1-6.8%, with the Accu-Chek performing best. All meters measured to 1.3 mmol/L with acceptable precision (coefficient of variation, <14%). Varying hematocrits between 0.2 and 0.7 L/L affected results of all meters. Interference at clinically relevant concentrations of galactose and possibly maltose was demonstrated with the Accu-Chek.

CONCLUSIONS

all three meters are sufficiently accurate and precise for in-hospital use. Because of possible interference by galactosemia or high hematocrit, the Accu-Chek is not the safest option for neonatal use. Patients receiving high doses of maltose in therapeutic infusions may still be at risk of being falsely classified as euglycemic or hyperglycemic with the reformulated Accu-Chek strips, and clinical evaluation of these strips in patients receiving maltose-containing infusions is urgently needed.

Familial isolated hyperparathyroidism is linked to a 1.7 Mb region on chromosome 2p13.3-14

BACKGROUND

Familial isolated hyperparathyroidism (FIHP) is an autosomal dominantly inherited form of primary hyperparathyroidism. Although comprising only about 1% of cases of primary hyperparathyroidism, identification and functional analysis of a causative gene for FIHP is likely to advance our understanding of parathyroid physiology and pathophysiology.

METHODS

A genome-wide screen of DNA from seven pedigrees with FIHP was undertaken in order to identify a region of genetic linkage with the disorder.

RESULTS

Multipoint linkage analysis identified a region of suggestive linkage (LOD score 2.68) on chromosome 2. Fine mapping with the addition of three other families revealed significant linkage adjacent to D2S2368 (maximum multipoint LOD score 3.43). Recombination events defined a 1.7 Mb region of linkage between D2S2368 and D2S358 in nine pedigrees. Sequencing of the two most likely candidate genes in this region, however, did not identify a gene for FIHP.

CONCLUSIONS

We conclude that a causative gene for FIHP lies within this interval on chromosome 2. This is a major step towards eventual precise identification of a gene for FIHP, likely to be a key component in the genetic regulation of calcium homeostasis.