IFCC recommendation on reporting results for blood glucose

In human beings, glucose is distributed like water between erythrocytes and plasma. The molality of glucose (amount of glucose per unit water mass) is the same throughout the sample. Different water concentrations in calibrator, plasma, and erythrocyte fluid can explain some differences that are dependent on sample type, methods requiring sample dilution, and direct reading biosensors detecting molality. Different devices for the measurement of glucose detect and report fundamentally different analytical quantities. The differences exceed the maximum allowable error of glucose determinations for diagnosing and monitoring diabetes mellitus, and they complicate the treatment. The goal of the International Federation of Clinical Chemistry, Scientific Division, Working Group on Selective Electrodes (IFCC-SD WGSE) is to reach a global consensus on reporting results. The document recommends harmonizing to the concentration of glucose in plasma (with the unit mmol/l), irrespective of sample type or technology. A constant factor of 1.11 will convert measured concentration in whole blood to the equivalent concentration in plasma.

Characterization of Bordetella pertussis strains of recent isolation

During the clinical trial conducted in Italy to evaluate the efficacy of new acellular pertussis vaccines, the most favorable conditions for the recovery and characterization of Bordetella pertussis strains, isolated from children with cough, were adopted. The nasopharyngeal aspirates were collected and sent to the laboratory in refrigerated conditions within 24 hours. Charcoal agar selective and non selective plates were used, and most of the isolates were recovered after 3-4 days of incubation. Confirmation of all suspected colonies included the use of biochemical tests and specific agglutination reaction with B. pertussis antiserum. Serotyping of fimbriae, susceptibility to erythromycin and DNA fingerprinting by Pulsed Field Gel Electrophoresis (PFGE), were performed to characterize B. pertussis isolates and to determine relatedness among different strains. Serotype 1,3 was the most represented in the bacterial population examined. A predominant pulsetype (PTA) characterized most of the isolates accounting for 71.4% of the strains examined. Eight subclones (23.5%) and three unrelated pulsetypes were also found. No resistant strains to erythromycin were detected.

Proposal for standardizing direct-reading biosensors for blood glucose

Direct-reading glucose biosensors sense molality (glucose per unit water mass) in the sample. With aqueous calibration, a direct-reading glucose biosensor produces higher results in blood and plasma than methods measuring concentration, theoretically by the ratio of water concentrations in calibrator and sample. To confirm this, we measured glucose in 14- blood and 40 plasma samples with the direct-reading glucose sensor in the Chiron Model 860 Blood Gas and Critical Analyte System and with our routine method (ESAT 6660; Eppendorf). The Chiron instrument is calibrated with a 10 mmol/L (180 mg/dL0 glucose calibrator (mass concentration of water = 0.99 kg/L). Assuming normal water concentrations of 0.84 and 0.93 kg/L in blood and plasma, respectively, we multiplied results from the Chiron sensor by 0.84/0.99 and 0.93/0.99 to obtain concentrations in blood and plasma. This conversion resulted in agreement of results with our routine method. An individual conversion based on hematocrit in each whole-blood sample was less satisfactory. To avoid confusion over variously measured and reported glucose results and reference values, we suggest standardization and reporting of whole-blood glucose results as equivalent plasma concentrations. This proposal may be conveniently achieved by using a commercially available reference material for glucose, NIST SRM 965.

Antibody kinetics and long-term sero-prevalence in the Italian clinical trial of acellular pertussis vaccines

In the Italian Pertussis Vaccine Trial, data were collected to evaluate the persistence of anti-pertussis antibodies. A sub-cohort of 1275 children was followed for this purpose until a mean age of 21 months. An additional evaluation included pooled cross-sectional analysis of serum specimens collected for analysis of cough illnesses. Antibodies to PT, FHA and PRN were measured by ELISA using a standardized technique. With both acellular vaccines in the study (the Chiron Biocine three-component and SmithKline Beecham three-component vaccines) there was a fast and steep decrease of mean geometric titres for PT, FHA and PRN in the months immediately following vaccination. Titres were close to the detection limit 15 months after the end of primary immunization. The immunogenicity of the whole-cell study vaccine (produced by Connaught Laboratories, Inc. Swiftwater, USA) was poor as determined one month after the third dose and no antibody was detected in nearly all children 15 months after the end of vaccination.

Frozen human serum reference material for standardization of sodium and potassium measurements in serum or plasma by ion-selective electrode analyzers

Three interlaboratory round-robin studies (RR1, RR2, and RR3) were conducted to identify a serum-based reference material that would aid in the standardization of direct ion-selective electrode (ISE) measurements of sodium and potassium. Ultrafiltered frozen serum reference materials requiring no reconstitution reduced between-laboratory variability (the largest source of imprecision) more than did other reference materials. ISE values for RR3 were normalized by the use of two points at the extremes of the clinical range for sodium (i.e., 120 and 160 mmol/L), with values assigned by the flame atomic emission spectrometry (FAES) Reference Method. This FAES normalization of ISE raw values remarkably improved all sources of variability and unified the results from seven different direct ISE analyzers to the FAES Reference Method value. Subsequently, a three-tiered, fresh-frozen human serum reference material was produced to the specifications developed in RR1-RR3, was assigned certified values for sodium and potassium by Definitive Methods at the National Institute of Standards and Technology (NIST), and was made available in 1990 to the clinical laboratory community as a Standard Reference Material (SRM); it is now identified as SRM 956. Albeit retrospectively, we show how applying an FAES normalization step identical to that used in RR4/5 to the ISE data for SRM 956 after the NIST Definitive Method values were known, consistently moved the ISE results for RR3 closer to the true value for Na+ and K+.

Frozen Human Serum Reference Material for Standardization of Sodium and Potassium Measurements in Serum or Plasma by Ion-Selective Electrode Analyzers

Three interlaboratory round-robin studies (RR1, RR2, and RR3) were conducted to identify a serum-based reference material that would aid in the standardization of direct ion-selective electrode (ISE) measurements of sodium and potassium. Ultrafiltered frozen serum reference materials requiring no reconstitution reduced between-laboratory variability (the largest source of imprecision) more than did other reference materials. ISE values for RR3 were normalized by the use of two points at the extremes of the clinical range for sodium (i.e., 120 and 160 mmol/L), with values assigned by the flame atomic emission spectrometry (FAES) Reference Method. This FAES normalization of ISE raw values remarkably improved all sources of variability and unified the results from seven different direct ISE analyzers to the FAES Reference Method value. Subsequently, a three-tiered, fresh-frozen human serum reference material was produced to the specifications developed in RR1-RR3, was assigned certified values for sodium and potassium by Definitive Methods at the National Institute of Standards and Technology (NIST), and was made available in 1990 to the clinical laboratory community as a Standard Reference Material (SRM); it is now identified as SRM 956. Albeit retrospectively, we show how applying an FAES normalization step identical to that used in RR4/5 to the ISE data for SRM 956 after the NIST Definitive Method values were known, consistently moved the ISE results for RR3 closer to the true value for Na+ and K+.

Design and preliminary performance characteristics of a newly proposed reference cell for ionized calcium in serum

A novel flow-through design for a calcium ion-selective liquid membrane and half-cell is proposed. The membrane is based on a neutral carrier for calcium and has the same quantitative chemical composition as that used in most commercial analyzers for ionized calcium (iCa2+). The membrane and half-cell follow the specifications in the Stage 1, Draft 4 document for a Reference Method for iCa2+ of the IFCC Expert Panel on pH, Blood Gases, and Electrolytes. Measurements of iCa2+ in serum with this cell confirm prior observations that protein adsorption on the membrane surface causes a rapid but reversible shift in the cell calibration signal and possible systematic errors in the measurement of iCa2+ in serum. When the protein adsorption phenomenon is maintained at equilibrium during calibrations and sample measurements, the new cell can be used to reproducibly assign iCa2+ concentrations to serum pools. These calibrated pools may then be used as reference materials and to resolve the biases known to exist among commercial iCa2+ analyzers.

Design and Preliminary Performance Characteristics of a Newly Proposed Reference Cell for Ionized Calcium in Serum

A novel flow-through design for a calcium ion-selective liquid membrane and half-cell is proposed. The membrane is based on a neutral carrier for calcium and has the same quantitative chemical composition as that used in most commercial analyzers for ionized calcium (iCa2+). The membrane and half-cell follow the specifications in the Stage 1, Draft 4 document for a Reference Method for iCa2+ of the IFCC Expert Panel on pH, Blood Gases, and Electrolytes. Measurements of iCa2+ in serum with this cell confirm prior observations that protein adsorption on the membrane surface causes a rapid but reversible shift in the cell calibration signal and possible systematic errors in the measurement of iCa2+ in serum. When the protein adsorption phenomenon is maintained at equilibrium during calibrations and sample measurements, the new cell can be used to reproducibly assign iCa2+ concentrations to serum pools. These calibrated pools may then be used as reference materials and to resolve the biases known to exist among commercial iCa2+ analyzers.