Artificially raised creatinine concentrations due to analytical interference for samples contaminated with total parenteral nutrition fluid.

BACKGROUND: A sample received in the laboratory from a patient receiving total parenteral nutrition (TPN) indicated that the patient may have renal dysfunction, but the results were not considered to be reliable enough to report. Investigations using a reference method for measurement of creatinine confirmed positive interference in the creatinine assay and distribution of samples via an External Quality Assessment (EQA) Scheme showed that this positive interference was method dependent. METHODS: Residual TPN fluid (Nutriflex Lipid Special) left in the bag after the patient had completed the infusion was collected and added to a patient serum pool in increasing amounts and distributed to different laboratories for analysis of creatinine and glucose through an EQA Scheme. RESULTS: Positive interference in a number of different creatinine assays was identified as a result of a component in the TPN fluid. Positive interference from high concentrations of glucose has been demonstrated to be a cause for falsely high results in Jaffe creatinine assays. CONCLUSIONS: The concern would be that a sample contaminated with TPN fluid would have both abnormal electrolytes and creatinine concentrations and give the impression that the patient was in renal failure due to analytical interference in the creatinine assay and laboratory staff need to be aware of this problem.

Use of copeptin in interpretation of the water deprivation test.

INTRODUCTION: Currently, the water deprivation test remains the standard method for distinguishing primary polydipsia (PP) from cranial diabetes insipidus (cDI) and nephrogenic diabetes insipidus (nDI). There is increasing interest in a direct estimate of antidiuretic hormone using plasma copeptin as a stable and reliable surrogate marker. We present our experience of measuring copeptin during the water deprivation test. METHODS: Forty-seven people (17 men) underwent a standard water deprivation test between 2013 and 2021. Plasma copeptin was measured at the start of the test and at the end of the period of water deprivation (maximum osmotic stimulation). Results were classified using prespecified diagnostic criteria. As it is known that a significant proportion of tests will reveal indeterminate results, a final diagnosis was obtained by including relevant pre- and post-test clinical criteria. This diagnosis was then used to plan individual treatment. RESULTS: Basal and stimulated copeptin were significantly higher in the nephrogenic DI group than other categories (p < .001). There was no significant difference in basal or stimulated copeptin between PP, cDI or partial DI. Nine results were indeterminate where the serum and urine osmolality did not give a unified diagnosis. Stimulated copeptin was helpful in reclassifying these patients into the final diagnostic groups. CONCLUSION: Plasma copeptin has additional clinical utility in interpretation of the water deprivation test and may continue to have a place alongside newer stimulation tests.

Relationship between faecal metronidazole and lactoferrin concentrations to clinical response of patients with Clostridioides difficile.

OBJECTIVES: We investigated patients with Clostridioides difficile-associated diarrhoea to see if clinical resolution correlated with faecal concentrations of metronidazole or markers of inflammation. METHODS: Faecal metronidazole, lactoferrin and serum CRP were measured daily. These were then compared with clinical progress. RESULTS: Metronidazole concentration correlated with lactoferrin (ρ = 0.17, p = 0.015), CRP (ρ = 0.23, p < 0.001) and number of diarrhoeal stools per day (ρ = 0.29, p < 0.001). Lactoferrin correlated with CRP (ρ = 0.57, p < 0.001) and the number of diarrhoeal stools per day (ρ = 0.52, p < 0.001) as did CRP (ρ = 0.52, p < 0.001). CONCLUSIONS: We found no association between cessation of diarrhoea and metronidazole or lactoferrin concentrations. There was a relationship between metronidazole concentrations and markers of inflammation and stool frequency.

Method for measurement of serum copper, zinc and selenium using total reflection X-ray fluorescence spectroscopy on the PICOFOX analyser: Validation and comparison with atomic absorption spectroscopy and inductively coupled plasma mass spectrometry.

BACKGROUND: Total reflection X-ray fluorescence is a comparably new method for the measurement of trace elements in biological samples. METHODS: Results obtained by total reflection X-ray fluorescence were compared to atomic absorption spectroscopy for Cu and Zn and inductively coupled plasma mass spectrometry for Cu, Zn and Se in patient serum. The total reflection X-ray fluorescence assay was characterized for accuracy; recovery; intra- and inter-assay imprecision (using patients' samples, external quality assurance and quality control materials); limit of blank; limit of detection; linearity; interference and stability of prepared samples. RESULTS: Minimal sample preparation is required for total reflection X-ray fluorescence and simultaneous multi-elemental analysis is possible in clinical samples. There was a small positive bias for Cu and Zn measurements using total reflection X-ray fluorescence compared to atomic absorption spectroscopy and inductively coupled plasma mass spectrometry and a significant negative bias for Se measurements by total reflection X-ray fluorescence relative to inductively coupled plasma mass spectrometry. Recovery, imprecision and linearity were acceptable. The limit of detection was shown to be 1.2 µmol/L for serum Cu, 1.8 µmol/L for serum Zn and 0.2 µmol/L for serum Se. CONCLUSIONS: Measurement of Cu and Zn in serum samples using total reflection X-ray fluorescence would be a viable alternative to atomic absorption spectroscopy or inductively coupled plasma mass spectrometry. The volatility of some Se compounds results in lower Se results being reported using total reflection X-ray fluorescence and further work would be necessary to identify whether total reflection X-ray fluorescence has an acceptable clinical sensitivity and specificity for the assessment of Se deficiency. Measurement of copper, zinc and selenium on whole blood samples is possible using total reflection X-ray fluorescence which may provide a more accurate assessment of trace element deficiency for patients with an acute phase response.

Development and validation of a liquid chromatography tandem mass spectrometry assay for the measurement of faecal metronidazole.

BACKGROUND: Metronidazole is an oral antibiotic which is widely used in the treatment of patients with Clostridium difficile associated disease. METHODS: This article describes the validation of a LC-MS/MS assay for the measurement of metronidazole in human faecal samples. RESULTS: Matrix matched and aqueous standards showed no significant difference in performance for the routine calibration of the assay. D4 deuterated metronidazole internal standard eluted with a different retention time to the undeuterated metronidazole on chromatography, hence zidovudine was used as an internal standard. Ion suppression was noted for both metronidazole and zidovudine due to unidentified compounds present in the faecal matrix and this was improved by extracting a smaller quantity of faeces and diluting the extract prior to analysis. Measurement uncertainty was 13% at 28,400ng/ml, 7.2% at 3300ng/ml, 3.9% at 320ng/ml, 13.6% at 109ng/ml and 30.9% at 20ng/ml. The assay was shown to be linear on dilution and the sensitivity of the assay was superior to HPLC assays using UV detection. The limit of detection was 5ng/ml, the limit of quantitation was 66ng/ml and the upper limit of the working range was 30,000ng/ml. Patient samples were stable at -20°C for 12months and extracted faecal samples were stable on storage for 1week at 4°C. There were no specific requirements for patient preparation or time of sample collection relative to taking metronidazole. CONCLUSIONS: Metronidazole can be quantified in faecal samples using LC-MS/MS which opens up opportunities for further research in this area.

Norethisterone interference in testosterone assays.

BACKGROUND: This article describes three patients in whom measured serum testosterone concentrations were found to be artifactually high due to interference from norethisterone medication. This interference was investigated further by distributing samples containing norethisterone through an external quality assessment scheme. METHODS: Serum samples containing different concentrations of norethisterone were distributed to participants in the UK external quality assessment scheme (UK NEQAS) for female testosterone in order to assess the degree of interference from norethisterone in different commercially available immunoassay and liquid chromatography-tandem mass spectrometry methods for measurement of testosterone. RESULTS: The results have shown that apparent serum testosterone concentrations in excess of 5 nmol/L may be obtained using the Roche E170 Modular immunoassay method for samples collected from patients taking norethisterone medication and this interference can be reproduced by adding norethisterone to serum samples. CONCLUSIONS: Although the biggest interference is seen with the Roche system, there is a small effect in the Siemens ADVIA Centaur assay and some of the other immunoassays may also be affected to a much lesser extent. Norethisterone does not interfere in testosterone measurements obtained by liquid chromatography-tandem mass spectrometry.

An immunoglobulin G complexed form of thyroid-stimulating hormone (macro thyroid-stimulating hormone) is a cause of elevated serum thyroid-stimulating hormone concentration.

BACKGROUND: Macrocomplexes can be the cause of elevated serum hormone concentrations and may cause diagnostic confusion. This is well recognized for prolactin and commonly screened for using polyethylene glycol (PEG) precipitation. The phenomenon and a suitable screening method is less familiar with respect to thyroid-stimulating hormone (TSH). METHOD: Samples sent to the laboratory for routine analysis of thyroid function and found to have a TSH >10 mU/L were evaluated to determine the prevalence of macro-TSH in the Roche Elecsys assay, using PEG precipitation with confirmation by gel filtration chromatography. RESULTS: Of 495 samples tested, 3 (0.6%) were found to have macro-TSH. From the distribution of recoveries, a cut-off <25% was determined for identifying samples requiring further investigation for the presence of macro-TSH. CONCLUSION: The prevalence of elevated TSH due to macro-TSH was found to be 0.6%. Laboratories should be aware of this cause of assay interference.

An IgG complexed form of vitamin B12 is a common cause of elevated serum concentrations.

OBJECTIVES: To report detailed investigations in a case of elevated serum B(12) due to the presence of an IgG-B(12) complex, to determine the prevalence of this phenomenon and to review the literature. DESIGN AND METHODS: 431 samples with elevated B(12) (median 1250 ng/L, range 901-114,480 ng/L) were treated with polyethylene glycol to precipitate immunoglobulin complexes. Samples with >50% of precipitable B(12) (PPB(12)) were further investigated by protein G adsorption, gel filtration chromatography and measurement of B(12) on different analytical platforms. RESULTS: Median PPB(12) was 22.6%, but in 35 samples (8.1%), median PPB(12) was more than 50%. Investigation of 27 of these samples with protein G-Sepharose confirmed the presence of an IgG-B(12) complex in 24, and in 15 cases, B(12) fell to within the reference range. After treatment of serum with reagents releasing B(12) from binding proteins, immunoreactivity co-eluted with free B(12). Immunoreactivity of the IgG-bound form of B(12) was confirmed using five B(12) assays in common use in the UK. CONCLUSIONS: At least 8% of samples with elevated vitamin B(12) contain an immunoglobulin complexed form of circulating B(12); this possibility should be considered in the interpretation of results.

Fecal dimeric M2-pyruvate kinase (tumor M2-PK) in the differential diagnosis of functional and organic bowel disorders.

BACKGROUND: Fecal inflammatory markers have been shown to be useful as noninvasive screening tools to differentiate patients with functional from organic bowel pathology. Of these markers calprotectin has been the most intensively studied. More recently, the dimeric isoform of M2-pyruvate kinase (tumor M2-PK) has been suggested as a marker of gastrointestinal inflammation. The aim of this study was to investigate fecal tumor M2-PK in the differentiation of functional from organic bowel disease. METHODS: Fecal calprotectin and tumor M2-PK were measured in 94 controls and 105 gastroenterology outpatients with a possible diagnosis of organic bowel disease. The diagnosis was made by clinical, endoscopic, and radiological criteria. RESULTS: Organic bowel disease was diagnosed in 14 patients (13%). Median calprotectin and tumor M2-PK concentrations were 24.5 microg/g and 1 U/mL in controls, 23 microg/g and 1 U/mL in functional, and 227.5 microg/g and 12.6 U/mL in organic bowel disease. Sensitivity, specificity, and positive and negative likelihood ratios for diagnosis of organic bowel disease were 93%, 92%, 11.6, and 0.07 for calprotectin and 67%, 88% 5.6, and 0.18 for tumor M2-PK, respectively. Calprotectin in combination with tumor M2-PK gave a sensitivity of 64%, specificity of 98%, and likelihood ratios of 32 and 0.03. Elevated calprotectin or tumor M2-PK decreased specificity to 87%, but increased sensitivity to 100%. CONCLUSIONS: Tumor M2-PK is able to differentiate organic from functional bowel disease but has a lower sensitivity, specificity, and predictive value than calprotectin. Further studies are required, alone or in combination with other markers, before its usefulness in this setting can be recommended.

Detection of haemolysis and reporting of potassium results in samples from neonates.

BACKGROUND: In vitro haemolysis is a common occurrence in clinical laboratories and causes a spurious increase in potassium. In the past, haemolysis was sought by visual inspection but is now commonly detected by automated measurement of the haemolytic index (HI). This study compared detection of haemolysis in adult and neonatal samples by inspection and measurement of HI and verified that a single equation is appropriate to correct for the increase in potassium in both haemolysed samples. METHODS: Laboratory staff inspected samples for haemolysis and their observations were compared with the measured HI. The potassium concentrations and haemolytic indices of 613 adult and 523 neonatal samples were correlated to derive equations to compensate for the increase in potassium with increase in HI. These were found not to differ significantly and a single equation for use in both populations was derived. RESULTS: The presence of icterus was found to decrease ability to detect haemolysis on inspection. The mean (95% confidence limits) potassium increase per unit HI was 0.0094 mmol/L (0.0078-0.0103 mmol/L) for adults and 0.0108 mmol/L (0.0094-0.0121 mmol/L) for neonates. The equation developed to compensate for potassium release in haemolysed samples was: adjusted potassium = measured potassium - (HI in micromol/L x 0.01). CONCLUSION: The use of HI rather than visual inspection is particularly recommended in neonates whose serum tends to be icteric. It can be used in the same correction equation as in adults to compensate for potassium released due to haemolysis and facilitate reporting a qualitative comment to assist in immediate clinical management.

Distribution of adiponectin, leptin, and metabolic correlates of insulin resistance: a longitudinal study in British children; 1: Prepuberty (EarlyBird 15).

BACKGROUND: The emergence of type 2 diabetes in young populations has mirrored a rising prevalence of obesity and insulin resistance during childhood and adolescence. At the same time, the role of adipokines as links between obesity and insulin resistance is becoming more appreciated. We sought to establish age- and sex-specific distributions of metabolic correlates of insulin resistance in healthy prepubertal children. METHODS: We collected fasting blood samples from a contemporary cohort of 307 British children at ages 5, 6, 7, and 8 years and measured insulin, glucose, triglycerides, total and HDL cholesterol, urate, glycohemoglobin, sex hormone-binding globulin (SHBG), leptin, and adiponectin. We used homeostasis model assessment (HOMA 2) to estimate insulin sensitivity (HOMA-%S) and beta-cell function (HOMA-%B). Anthropometric measures included body mass index. RESULTS: Body mass index increased from age 5 to 8 years (P < 0.001). HOMA-%B decreased (P < 0.001) and HOMA-%S increased (P < 0.05), but glucose also increased (P < 0.001) whereas glycohemoglobin decreased (P < 0.001). Consistent with the rise in insulin sensitivity, HDL cholesterol increased (P < 0.001) and triglycerides decreased (NS), whereas adiponectin decreased (P = 0.02). The patterns were similar in boys and girls, although girls were less insulin sensitive throughout. Accordingly, triglycerides tended to be higher in the girls, and HDL cholesterol and SHBG lower. CONCLUSIONS: The metabolic disturbances associated with insulin resistance appear to be more advanced in girls. Markers of metabolic health improve in both sexes from 5 to 8 years, despite rising adiposity.

Cerebrospinal fluid: the role of biochemical analysis.

Biochemical analysis of cerebrospinal fluid may provide answers to important clinical questions. This review summarizes these questions and outlines the value and limitations of cerebrospinal fluid analysis.

Successful rescue of severe hypernatraemia (196 mmol/L) by treatment with hypotonic fluid.

Hypernatraemia over 160 mmol/L is considered to be severe. This case reports a patient who developed extreme hypernatraemia with a serum sodium concentration of 196 mmol/L. The patient was known to have chronic renal impairment and was admitted with acute deterioration of renal function secondary to dehydration. This was considered to be secondary to poor oral fluid intake (related to depression) and lithium-induced nephrogenic diabetes insipidus with salt-losing nephropathy. The patient had a high urinary sodium excretion but was also in a pure water losing state as evidenced by an inappropriately low urine osmolality for the plasma osmolality and was successfully treated with hypotonic intravenous fluid and desmopressin.