Multicenter comparison of Etest, Vitek2 and BD Phoenix to broth microdilution for beta-lactam susceptibility testing of Streptococcus pneumonia.

PURPOSE: To assess performance of Etest®, Vitek®2 and BD Phoenix™ to determine the susceptibility of Streptococcus pneumoniae strains to penicillin, ampicillin and cefotaxime. METHODS: Sixty unique S. pneumoniae challenge strains were selected to cover a wide range of penicillin, ampicillin and cefotaxime minimal inhibitory concentrations (MICs). Strains were analyzed in four different Belgian laboratories. Etest® benzylpenicillin (BEN), ampicillin/amoxicillin (AMP) and cefotaxime (CTA) (bioMérieux), Vitek®2 AST-ST03 (bioMérieux) and BD Phoenix™ SMIC/ID-11 testing were each performed in two different labs. Results were compared to Sensititre® broth microdilution (BMD) (Thermo Fisher Scientific) results. MIC results were interpreted using EUCAST non-meningitis breakpoints (v 13.0). RESULTS: Essential agreement (EA) was ≥ 90% for all methods compared to BMD, except for Etest® BEN on Oxoid plate (58.3%), Etest® AMP (both on Oxoid (65.8%) and BD BBL plate (84.2%)). Categorical agreement (CA) for penicillin was only ≥ 90% for Vitek®2, for other methods CA ranged between 74 and 84%. CA for AMP was for all methods < 90% (range 75.8-88.3%) and CA for CTA was between 87 and 90% for all methods except for Etest on Oxoid plate (79.2%). CONCLUSIONS: Our study indicates that Vitek®2 and BD Phoenix™ are reliable for providing accurate pneumococcal susceptibility results for BEN, AMP and CTA. Using Etest BEN or AMP on Oxoid plate carries a risk of underestimating the MIC and should be interpreted with caution, especially when the obtained MIC is 1 or 2 doubling dilutions below the S or R clinical breakpoint.

Serum free light chain analysis: persisting limitations with new kids on the block.

OBJECTIVES: Serum free light chain (sFLC) measurements have inherent analytical limitations impacting sFLC clinical interpretation. We evaluated analytical and diagnostic performance of three polyclonal sFLC assays on four analytical platforms. METHODS: sFLC concentration was measured using Diazyme FLC assays (Diazyme) on cobas c501/c503 analyzer (Roche); Freelite assays (The Binding Site) on Optilite analyzer (The Binding Site) and cobas c501 analyzer and Sebia FLC ELISA assays (Sebia) on AP22 ELITE analyzer (DAS). Imprecision, linearity, method comparison vs. Freelite/Optilite, antigen excess detection and reference value verification were assessed. Diagnostic performance was compared on 120 serum samples and on follow-up samples of five patients with κ and λ monoclonal gammopathy. RESULTS: Method comparison showed excellent correlation with Freelite/Optilite method for all assays. A large proportional negative bias was shown for both Sebia κ and λ ELISA and a significant positive proportional bias for λ in the low (<10 mg/L) Freelite/cobas c501 method. Clinically relevant underestimation of κ sFLC levels due to antigen excess was shown for 7% of each Diazyme/cobas application and for 11 and 32.1% of λ sFLC assay of respectively Diazyme/cobas and Sebia/AP22. sFLC reference values revealed application specific. Cohen's κ values were (very) good for κ sFLC but only moderate to good for λ sFLC. In 4/10 follow-up patients, significant differences in clinical interpretation between sFLC assays were noticed. CONCLUSIONS: Important analytical limitations remain for all sFLC applications. Differences in reference values and diagnostic performance hamper interchangeability of sFLC assays. Assay specific sFLC decision guidelines are warranted.

Concordance of apolipoprotein B concentration with the Friedewald, Martin-Hopkins, and Sampson formulas for calculating LDL cholesterol.

INTRODUCTION: Two new formulas, the Martin-Hopkins and the Sampson formula, were recently developed to overcome shortcomings of the Friedewald formula for calculating LDL-cholesterol. We aimed to compare the concordance of the two formulas with apolipoprotein B (apoB), a surrogate marker of the number of LDL particles. MATERIALS AND METHODS: In a study of serum lipid data of 1179 patients who consulted the AZ St-Jan Hospital Bruges for cardiovascular risk assessment, the correlation and concordance of the Friedewald, Martin-Hopkins and Sampson formulas with apoB concentration, measured by immunonephelometry, were determined and compared. RESULTS: The Martin-Hopkins formula showed significantly higher correlation coefficient than the Friedewald formula with apoB in the entire dataset and in patients with low LDL-cholesterol < 1.8 mmol/L. Both Martin-Hopkins and Sampson formulas yielded > 70% concordance of LDL-cholesterol with regard to treatment group classification based on population-equivalent thresholds of apoB in hypertriglyceridemic patients (2-4.5 mmol/L), with the highest concordance (75.6%) obtained using Martin-Hopkins formula vs. 60.5% with Friedewald formula. CONCLUSION: The Martin-Hopkins (and, to a lesser extent, Sampson) formula is more closely associated with the number of LDL particles than Friedewald formula. This, in combination with literature evidence of lesser accuracy of the Friedewald formula, is an argument to switch from Friedewald to a modified, improved formula.