Albumin and calcium reference interval using healthy individuals and a data-mining approach.

BACKGROUND: Harmonization of reference intervals for analytes that have a sound calibration and metrological traceability is a widely recommended practice. The UK Pathology Harmony has recently harmonized reference intervals for calcium and albumin. In this study, we have determined the reference intervals for calcium and albumin on the UK's most commonly used analytical platforms. METHOD: A prospective reference population of healthy individuals was recruited according to the IFCC CRIDL criteria. A second indirect population was collected from 14 primary care setting and measured in laboratories using various analytical platforms and methods (Roche, Abbott, Beckman and Siemens analytical platforms). RESULTS: In total, 299 subjects were recruited; the central 95th centile values for calcium for three out of four analytical platforms were in a close agreement with UK Pathology Harmony reference intervals of 2.2-2.6 mmol/L. Reference intervals of BCG methods from both cohorts and irrespective of analytical platforms were higher for both lower and upper reference limits than those for BCP. In comparison, the indirect study showed an age-related variation. The younger population reference intervals varied by up to 5.7% at the lower reference limit and up to 12% at the upper reference limit compared with Pathology Harmony reference intervals, and the older population showed a variation of up to 14% at both limits. CONCLUSION: While calcium reference intervals can be a subject for harmonization, albumin reference intervals studied showed large variation which is unsupportive of embracing a common reference interval for albumin.

The effect of different analytical platforms and methods on the performance of population-specific adjusted calcium equation.

BACKGROUND: A recent attempt to improve the diagnostic value of adjusted calcium addressed a primary care-specific adjusted calcium equation, but validated the new equation for Roche Cobas, BCG and NM-BAPTA methods only. In this study, we aim to validate a population-specific equation for other methods and platforms. METHOD: We collected retrospective patient data-sets from 15 hospital laboratories using a range of commercially available analytical platforms and methods for calcium and albumin measurements. Raw data-sets were collected and filtered according to Payne's criteria, and separate adjusted calcium equations were derived for hospitalized and primary care patients. RESULTS: Mean albumin and calcium results were significantly higher in primary care populations (P < 0.0001). The prevalence of hypocalcaemia using adjusted calcium ranged between 6% and 44% for inpatient data-sets and was higher in users of BCG methods. The application of community-specific adjustment equation to primary care data-sets reduced the prevalence of hypocalcaemia (mean 1.7%, range 0.8-3.7%). CONCLUSION: We demonstrated that the use of a community-specific calcium adjustment equation to a primary care population reduces both the percentage and the variation of hypocalcaemia between different laboratories.

The effect of serum matrix and gender on cortisol measurement by commonly used immunoassays.

BACKGROUND: Considerable intermethod bias has been observed between cortisol immunoassays, with some also displaying a gender difference. Cortisol immunoassay performance is affected by serum matrix effects such as changes in steroid binding proteins and presence of interfering steroids which can be altered in various clinical settings. This study investigates cortisol immunoassay bias in pregnancy, renal failure and intensive care patients. METHODS: Serum remaining after routine analysis from pregnant patients, patients on the intensive care unit and patients with renal failure were obtained prior to disposal and used to create 20 anonymous samples per group. A male and female serum pool was prepared and spiked with cortisol. Samples were aliquoted and distributed to four hospitals for cortisol analysis by immunoassays from four different manufacturers. Cortisol was also measured by an isotope dilution-gas chromatography-mass spectrometry method for comparison of assay bias. RESULTS: Differences in cortisol immunoassay bias were observed across the different patient groups. A negative bias compared to pooled serum samples was observed for pregnancy serum, whilst a more positive bias was seen in renal failure and intensive care patients. Variation in bias was greatest in renal failure with the Roche E170 the most affected and the Abbott architect the least (interquartile ranges 44% and 14%, respectively). CONCLUSIONS: Cortisol immunoassay bias may be affected by gender and differences in serum matrix from patients with various clinical conditions. Users of cortisol assays should be aware of differing matrix effects on their assay and the relevance of these for the interpretation of clinical results.

Do anxiety or hypocapnia predispose to apnoea after induction of anaesthesia?

We have studied the incidence of apnoea after induction of anaesthesia in patients allocated randomly to receive a standardized dose of either propofol or etomidate. We measured anxiety before operation with a standard questionnaire and end-tidal carbon dioxide concentration from a mask during breathing 35% oxygen, before induction of anaesthesia. Respiration was measured by pneumotachograph and impedance pneumograph. There was no significant relationship between anxiety score and end-tidal carbon dioxide concentration before operation. Patients given propofol (n = 26) received a median dose of 157 mg over 70 s, and 17 became apnoeic (median duration 24 s, quartile values 0, 76). Apnoea was more severe in patients whose preoperative end-tidal carbon dioxide value was less than the median value (median duration of apnoea 61 s compared with 10 s; P < 0.05). Patients given etomidate (n = 25) received 16.2 mg in 57 s, which was a significantly smaller fraction of the calculated dose requirement, and had significantly less apnoea: eight became apnoeic (median duration 0 s, quartile values 0, 23 s). There was no relationship between apnoea and end-tidal carbon dioxide concentration in these patients. Anxiety did not relate to the incidence of apnoea with either induction agent. We conclude that apnoea after induction of anaesthesia with propofol is more likely if hypocapnia is present but we could not relate apnoea or hypocapnia to anxiety in the ward before operation.