False Laboratory Test Result Through Colistin Interference in an Intensive Care Patient: Case Report

Investigation of the effect of vitamin K1 prophylaxis on newborn screenings tests in newborns

Background

Routine screening for hereditary disorders in newborns includes screening for treatable metabolic and endocrine disorders, such as biotidinase deficiency, galactosemia, maple syrup urine disease, hypothyroidism, and cystic fibrosis. Incorrect test results may be encountered due to the use of vitamin K1. To investigate the interference effect of vitamin K1 on neonatal screening tests and to raise awareness of erroneous measurements.

Methods

Heel blood samples were taken from 25 newborns born in a neonatal intensive care unit. Dry blood C0, C2, C3, C4, C4DC, C5:1, C5OH, C5DC, C6, C6DC, C8, C8:1, C8DC, C10, C10:1, C10DC, C12, C14, C14:1, C14:2, C16, C16:1, C18, C18:1, C18:2, C18:OH, methylglutaryl, valine, leucine/isoleucine, methionine, phenylalanine, argininosuccinic acid, aspartate, alanine, arginine, citrulline, glycine, ornithine, and glutamate tests were studied using the tandem mass spectrometry (MS) method. The results of the heel blood samples obtained before and after the application of vitamin K1 (Phyto menadione) were compared.

Results

In two studies conducted with in vitro and in vivo tests, C0, C2, C3, C4, C4DC, C5, C5OH, C6, C8, C10, C10:1, C14, C16, C16:1, C18, C18:1, methylglutaryl, phenylalanine, argininosuccinic acid, tyrosine, aspartate, arginine, citrulline, glycine, and glutamine were all significantly elevated (p < 0.05).

Conclusions

Heel blood samples may yield false results due to vitamin K1 administration. In the case of doubtful results, a new sample should be taken and the measurement should be repeated.

Pre-analytical mysteries: A case of severe hypervitaminosis D and mild hypercalcaemia

We describe a case of severe hypervitaminosis D and mild hypercalcaemia in a 68-year-old woman who presented with fatigue and weight loss. Her 25-hydroxy vitamin D (25OHD) was > 400 nmol/L (50-150) and corrected serum calcium was 2.83 mmol/L (2.1-2.6). Her intact parathyroid hormone (PTH) was 4.9 pmol/L (2.0-9.5). Further investigation revealed an IgM kappa paraprotein, and a bone marrow aspirate confirmed a diagnosis of lymphoplasmacytic lymphoma/Waldenstrom’s macroglobulinemia (LPL/WM). As the vitamin D level was discordant with the patient’s other results and presentation, the presence of an assay interferent was suspected. A 1-in-2 dilution of the sample returned a 25OHD result of 84 nmol/L in keeping with the presence of an interferent. Testing for rheumatoid factor was negative. The sample was treated with an antibody blocking reagent (Scantibodies) and results were not consistent with heterophile antibody interference. The sample was then analysed using liquid chromatography tandem mass spectrometry (LC-MS/MS), which returned a 25OHD result of 82 nmol/L. Testing on an alternative immunoassay platform produced a 25OHD result of 75 nmol/L. Reapeted testing on the original platform following reduction of the monoclonal paraprotein with chemotherapy, returned a result of 64 nmol/L. The patient’s mild hypercalcaemia persisted following resolution of the monoclonal paraprotein, in keeping with a diagnosis of primary hyperparathyroidism. This case highlights the potential for paraproteins to cause assay interference, and the importance of considering interference when results are incongruous with the clinical presentation.