COMPANY PROFILE

STT ConsultingAbraham Hansstraat 11

B-9667 Horebeke, Belgiume-mail: dietmar@stt-consulting.com; Tel + FAX: +32/5549 8671

WHAT IS STT ConsultingSTT Consulting is a consultancy-bureau, established by Dietmar Stöckl, PhD.

EXPERTISE OF STT Consulting (see also list of publications, lectures, etc.)

STT Consulting has expertise for internal quality control and external quality assessment (EQA). As member of several European Working Groups, Dietmar elaborated guidelines for internal quality control and concepts for modern EQA (in particular: matrix-free assessment of test-system bias by use of single donations bearing reference method target values).

STT Consulting offers unique expertise related to reference methods and standardization in clinical chemistry. This expertise stems from the work of Dietmar at the Institut für Standardisierung und Dokumentation im medizinischen Laboratorium eV (INSTAND), where he built-up and headed the reference laboratory for more than 7 years (Note: INSTAND is one of the two official reference institutions that are responsible for the German EQA-system). It stems, further, from the scientific cooperation with Prof. Dr. Linda M. Thienpont from the University of Gent. During this cooperation, reference methods for t-electrolytes, organic substrates, steroid hormones, and other analytes were developed and implemented in the respective reference laboratories.

STT Consulting, further, has expertise for development of accuracy-based routine methods for clinical chemistry. On one hand, this expertise is based on scientific publications regarding desirable analytical specifications for routine methods. On the other hand, it is based on practical experience gained with evaluations of routine methods with reference methods. In addition, STT Consulting is developing, together with Scantibodies Laboratories and the University of Ghent, panels of single donation native sera certified with reference methods (SysRef) for routine method evaluation/calibration/verification.

STT Consulting has excellent contact to university research. Dietmar is associated to the laboratory of Prof. Thienpont as free researcher (main research projects: development of reference methods based on isotope dilution gas or liquid chromatography chromatography/mass spectrometry).

STT Consulting, other relevant expertise: Metrology, graphical & statistical tools for method comparison

“About us”

WHICH SERVICES ARE OFFERED BY STT Consulting?

Services of STT Consulting are offered in the form of: Consulting Course manuals, Books, Software templates Lectureship/Seminars/Courses Active help in set-up of reference laboratories.

WHO CAN PROFIT FROM SERVICES OFFERED BY STT Consulting?

Routine laboratories: Internal quality control Quality assurance Accreditation Laboratory statistics

Manufacturers of in vitro diagnostic medical devices: “In-house” training internal quality control Implementation of traceability according to the Directive 98/79/EC Standardization of methods Method evaluation with reference methods and/or reference materials Knowledge transfer from university research

Reference laboratories: Set-up and application of reference methods

EQA organizers: Development of surveys that apply reference method target values and native sera

Government: Consultancy for the implementation of traceability according to the Directive 98/79/EC

FULL COORDINATES OF STT Consulting

STT Consulting, Dr. Dietmar StöcklAbraham Hansstraat 11, B-9667 Horebeke (Belgium)Website: www.stt-consulting.come-mail: dietmar@stt-consulting.comTel. & FAX: +32/55 49 86 71Bank: ASLK Gent, Zwijnaardsesteenweg 674, B-9000 Gent (Belgium),Account No: 001-1148958-69BTW (VAT): BE 787 056 515; Register: HRK: H/000 40782

Selected scientific activities

Working groups Member of: European External Quality Assessment Organizers Working Group A:

Analytical goals in Laboratory Medicine. 1991-1996. Member of: European External Quality Assessment Organizers Working Group B:

Target Values in EQAS. 1991-1996. Member of: Analytical Quality in the Medical Laboratory. An international working

group. Established: 1998. Advisor: 4th. Bergmeyer Conference, Proposals for two immunomethod reference

systems: cortisol and human chorionic gonadotropin. "Standardization of haptene immunoprocedures: total cortisol". Lenggries, November 1992.

Chairman: Working Group 1: "Definitions related to analytical quality specifications, such as limit of detection (LD), limit of quantification (LQ), sensitivity, etc". At the "Second E2 International Workshop Brugge '95" (16.11 - 18.11.95, Brugge, Belgium).

Courses/Symposia/Info-days Co-organizer and lecturer: Postacademical education: "Special topics in quantitative

biomedical analysis" (academical year 1994/5). In cooperation with Prof. AP De Leenheer and Prof. LM Thienpont (University of Ghent).

Co-organizer and lecturer: Continuous education: “Analytical quality in the medical laboratory: Concepts for method selection, evaluation, and control”. In cooperation with Westflemish Association of Clinical Biochemists (Waregem, Belgium, 1997).

Co-organizer and lecturer: Continuous education: “Analytical quality in the medical laboratory: Concepts for method selection, evaluation, and control”. In cooperation with Belgian Association of Laboratory Technologists and Hogeschool Gent (Gent, Belgium, 1998).

Co-organizer: Postacademical education: "Practice-oriented strategies for the development and evaluation of analytical methods. FOCUS: Graphical and statistical techniques for the interpretation of method comparison studies" (academical year 2000/1). In cooperation with Prof. LM Thienpont (University of Ghent).

Co-organizer: Postacademical education: "Analytical quality in laboratory medicine – The laboratory in the field of tension between commercially available and desirable quality“ (academical year 2001/2). In cooperation with Prof. LM Thienpont (University of Ghent).

Co-organizer: Information day: “Analytical quality in laboratory medicine - the manufacturer in the field of tension between "desirable" quality and economically justified quality. Roche Diagnostics Belgium (October 2001). In cooperation with Prof. LM Thienpont (University of Ghent) and Roche Diagnostics Belgium.

Co-organizer and lecturer: Symposium: “Graphical and statistical techniques for the interpretation of method comparison studies". 14th IFCC European Congress of Clinical Chemistry and Laboratory Medicine - Euromedlab (2001, Prague, Czech Republic).

Co-organizer and lecturer: Educational course on biostatistics. 18th International Congress of Clinical Chemistry and Laboratory Medicine (Kyoto, Japan, 2002).

Co-organizer and lecturer: Statistical and graphical tools for the medical laboratory – A problem oriented journey from test utility to internal quality control (Slovakia, September 2003).

Lectures Development of criteria for the evaluation of reference method values. IX European

Congress of Clinical Chemistry (1991, Krakow, Poland). Consequences of Reference Method target values in External Quality Assessment.

First National Symposium of the Czech Society of Clinical Chemistry (1993, Sec, Czech Republic).

Use of standard reference materials. ACB Regional meeting (Glasgow, Scotland, 1995).

Uncertainty and error - two conflicting concepts for basic metrological terms in laboratory medicine? Labquality Days. New Developments of Quality Systems for Medical Laboratories (1995, Helsinki, Finland).

The biological approach for specification of analytical quality. Laboratory Medicine '95. 11th IFCC European Congress of Clinical Chemistry. Satellite Meeting on Quality Assurance of Analytical Performances in Clinical Chemistry (1995, Tampere, Finland).

Ideal quality goals for quantities assayed in serum. International Congress on Automation and New Technology in the Clinical Laboratory. Satellite Symposium on Analytical Quality Assurance in the European Context (1995, Sitges, Spain).

Definitions related to analytical quality specifications, such as limit of detection (LD), limit of quantification (LQ), sensitivity, etc. Second E2 International Workshop Brugge '95 (Brugge, Belgium, 1995)

Test kit validation with reference methods. "Technology-day reference methods in laboratory diagnostics" (Bernried, Germany, 1995).

Reference materials – Systems for laboratory assurance and surveillance. Satellite meeting to the XVI International Congress of Clinical Chemistry 1996: Quality Systems in Clinical Chemistry (1996, Llandudno, Wales).

In Vitro Diagnostic Medical Devices: requirements for implementing the traceability concept. International Chemometric Conference (Chemometrics IV) (1996, Brno, Czech Republic).

Method (test-system) assessment in EQA: requirements for materials, targets, and data recruitment and treatment. International Chemometric Conference (Chemometrics IV) (1996, Brno, Czech Republic).

Modern Aspects of External Quality Assessment; Requirements for Materials, Targets, Limits and Data Recruitment and Treatment. Labquality days. International Quality Assessment Meeting for Medical Laboratories (Helsinki, Finland, 1997).

Traceability of clinical laboratory measurements - the problem of commutability of calibrators and reference materials. Conference of the Polish Society of Laboratory Diagnostics (Warsaw, Poland, 1997).

Application of traceable materials in the routine laboratory: Case studies. Pathology 2000, Quality Laboratory Open Seminar (2000, Birmingham, UK).

Validation of field methods in relation to quality specifications. Laboratory Medicine 2000 - XXVII Nordic Congress of Clinical Chemistry (2000, Bergen, Norway).

Graphical techniques for the intepretation of method comparison studies. Education days for Clinical Biochemists: Method validation. Odense, Denmark: 17-20 December 2001.

Types of errors and combinations. Quality management course for Clinical Biochemists. Vejle, Denmark: May 12-14 2003.

Internal quality control: error detection and error rejection. Quality management course for Clinical Biochemists. Vejle, Denmark: May 12-14 2003.

Statistical and graphical techniques for the intepretation of method comparison studies. Euromedlab 2003, Barcelona: June 2003.

PUBLICATIONS

Reference methodsElectrolytes (total and ionized)(1) Thienpont LM, Van Nuwenborg JE, Stöckl D. Ion chromatography as potential reference methodology for the determination of total calcium and magnesium in human serum. Anal Chem 1994;66:2404-8.(2) Thienpont LM, Van Nuwenborg JE, Stöckl, D. Ion chromatography as potential reference methodology for the determination of total sodium and potassium in human serum. J Chromatogr A 1995;706:443-50.(3) Thienpont LM, Van Nuwenborg JE, Reinauer H, Stöckl, D. Validation of candidate reference methods based on ion chromatography for determination of total-sodium, -potassium, -calcium, and -magnesium in serum through comparison with flame atomic emission and absorption spectrometry. Clin Biochem 1996;29:501-8.(4) Van Nuwenborg JE, Stöckl D, Thienpont LM. Practical aspects of suppressed ion chromatography for cations in human serum. J Chromatogr A 1997;770:137-41.(5) Van Nuwenborg JE, Stöckl D, Thienpont LM. State of the art of serum total calcium measurement as investigated by split-sample measurement with an ion chromatography candidate reference method. Eur J Clin Chem Clin Biochem 1997;35:297-300. (6) Thienpont LM, Van Nuwenborg JE, Stöckl D. Ion chromatography as reference method for serum cations [review]. J Chromatogr A 1997;789:557-68.(7) Thienpont LM, Van Nuwenborg JE, Stöckl D. Intrinsic and routine quality of serum total potassium measurements as investigated by split-sample measurement with an ion chromatography candidate reference method. Clin Chem 1998;44:849-57.(8) Van Nuwenborg JE, Thienpont LM, Stöckl D, Davies KW, Palicka V, Friedecký B, Beranek M, Kratochvíla J. European survey of serum potassium reference measurements. Ann Clin Biochem 1999;36:80-5. (9) Dewitte K, Stöckl D, Thienpont LM. Standardization of ionized magnesium by correlation with total magnesium [Letter]. Scand J Clin Lab Invest 1998;58:603-4.(10) Dewitte K, Stöckl D, Thienpont LM. Reliability of measurement of ionized magnesium in ultrafiltrate [Letter]. Clin Chem 1999;45:157-8; Erratum: 588-9.(11) Thienpont LM, Dewitte K, Stöckl D. Serum complexed magnesium - a cautionary note on its estimation and its relevance for standardization of ionized magnesium [Letter]. Clin Chem 1999;45:154-5.(12) Dewitte K, Stöckl D, Thienpont LM. pH dependency of serum ionised calcium. Lancet 1999;354:1793-4.(13) Dewitte K, Stöckl D, Van de Velde M, Thienpont LM. Evaluation of intrinsic and routine quality of serum total magnesium measurement. Clin Chim Acta 2000;292:55-68.

Substrates(1) Stöckl D, Reinauer H. Candidate Reference Methods for the determination of target values for cholesterol, creatinine, uric acid, and glucose in external quality assessment and internal accuracy control. I. Method setup. Clin Chem 1993;39:993-1000.(2) Thienpont LM, De Leenheer AP, Stöckl D, Reinauer H. Candidate Reference Methods for the determination of target values for cholesterol, creatinine, uric acid, and glucose in external quality assessment and internal accuracy control. II. Method transfer. Clin Chem 1993;39:1001-6.(3) Thienpont LM, Van Landuyt K, Stöckl D, De Leenheer AP. Candidate reference method for determining serum creatinine by isocratic HPLC: validation with isotope dilution gas chromatography-mass spectrometry and application for accuracy assessment of routine test-kits. Clin Chem 1995;41:995-1003.(4) Thienpont LM, Van Landuyt K, Stöckl D, De Leenheer AP. Evaluation of four frequently used test systems for serum cholesterol by an isotope dilution gas chromatography-mass spectrometry candidate reference method. Clin Chem 1996;42:531-5.

Steroid hormones(1) Stöckl D, Reinauer H, Thienpont LM, De Leenheer AP. Determination of aldosterone in human serum by isotope dilution gas chromatography/mass spectrometry using a new heptafluorobutyryl derivative. Biol Mass Spectrom 1991;20:657-64.(2) Thienpont LM, De Brabandere VI, Stöckl D, De Leenheer AP. Use of cyclodextrins for prepurification of progesterone and testosterone from human serum prior to determination with isotope dilution gas chromatography/mass spectrometry. Anal Chem 1994;66:4116-9.(3) Stöckl D, Thienpont LM, De Brabandere VI, De Leenheer AP. Novel perfluoroacyl derivatives of corticosteroids and related substances for potential use in quantitative gas chromatography mass spectrometry. J Am Soc Mass Spectrom 1995;6:264-76.(4) De Brabandere VI, Thienpont LM, Stöckl D, De Leenheer AP. Three routine methods for serum cortisol evaluated by comparison with an isotope dilution gas chromatography-mass spectrometry method. Clin Chem 1995;41;1781-3.(5) Thienpont LM, De Brabandere VI, Stöckl D, De Leenheer AP. Candidate reference method for determining serum cortisol based on isotope dilution-gas chromatography mass spectrometry using heptafluorobutyrilation as derivatisation method. Anal Biochem 1996;234:204-9.

Other analytes (1) Thienpont LM, Van Nieuwenhove B, Stöckl D, De Leenheer AP. Candidate reference method for the determination of serum theophylline and its application for target-setting in external quality assessment and routine method evaluation. Clin Chem 1994;40:1503-11.(2) Thienpont LM, De Brabandere VI, Stöckl D, De Leenheer AP. Development of a new method for the determination of thyroxine in serum based on isotope dilution-gas chromatography mass spectrometry. Biol Mass Spectrom 1994;23:475-82.(3) De Brabandere VI, Stöckl D, Thienpont LM, De Leenheer. On the use of trimethylchlorosilane in methanol for methylation of thyroxine prior to perfluoroacylation and isotope dilution gas-chromatography/mass spectrometry [Letter]. J Mass Spectrom 1998;33:1032.(4) De Brabandere VI, Hou P, Stöckl D, Thienpont LM, De Leenheer AP. Isotope dilution-liquid chromatography/electrospray ionization-tandem mass spectrometry for the determination of serum thyroxine as potential reference method. Rapid Commun Mass Spectrom 1998;12:1099-1103.(5) Fierens C, Thienpont LM, Stöckl D, Willekens E, De Leenheer AP. Quantitative analysis of urinary C-peptide by liquid chromatography-tandem mass spectrometry with a stable isotopically labelled internal standard. J Chromatogr A 2000;896:275-8.(6) Fierens C, Stöckl D, Thienpont LM, De Leenheer AP. Towards a reference method for serum free thyroxine based on isotope dilution mass spectrometry. Immunoanalysis Biol Spec 2000;15:311-7.(7) Fierens C, Stöckl D, Thienpont LM, De Leenheer AP. Strategies for determination of insulin with tandem electrospray mass spectrometry: implications for other analyte proteins? Rapid Commun Mass Spectrom 2001;15:1433-41.(8) Fierens C, Stöckl D, Baetens D, De Leenheer AP, Thienpont LM. Standardization of C-Peptide Measurements in Urine by Method Comparison with Isotope-Dilution Mass Spectrometry. Clin Chem 2003;49:992-4.

Concepts for development and application of reference methods in clinical chemistry(1) Stöckl D, Reinauer H, Appel W, von Klein-Wisenberg A. Diskussionsvorschlag für ein einheitliches Referenzmethodenkonzept auf der Grundlage der "Richtlinien der Bundesärztekammer zur Qualitätssicherung in medizinischen Laboratorien". Lab Med 1991;15:336-9.(2) Stöckl D, Reinauer H. Development of criteria for the evaluation of reference method values. Scand J Clin Lab Invest 1993;53 Suppl 212:16-8.(3) Thienpont LMR, Stöckl D. Reference methods and reference method values. Klin Biochem Metab 1994;2:4-9. (4) Thienpont LM, Stöckl D, De Leenheer AP. Isotope dilution mass spectrometry and implementation of a common accuracy base for routine medical laboratory analysis: Practice and prospects. J Mass Spectrom 1995;30:772-4. (5) Thienpont LM, Franzini C, Kratochvila J, Middle J, Ricós C, Siekmann L, Stöckl D. Analytical quality specifications for reference methods and operating specifications for networks of reference laboratories. Discussion paper from the members of the external quality assessment (EQA) Working Group B on target values in EQAS. Eur J Clin Chem Clin Biochem 1995;33:949-57.(6) Stöckl D, Franzini C, Kratochvila J, Middle J, Ricós C, Siekmann L, Thienpont LM. Analytical specifications of reference methods. Compilation and critical discussion (from the members of the European EQA-organizers Working Group B) [review]. Eur J Clin Chem Clin Biochem 1996;34:319-37.

Desirable analytical performance for routine methods in clinical chemistry(1) Stöckl D. Desirable performance criteria for quantitative measurements in medical laboratories based on biological analyte variation - hindrances to reaching some and reasons to surpass some. Clin Chem 1993;39:913-4.(2) Stöckl D. Desirable analytical quality specifications for clinical chemistry laboratories. Clin Chem 1994;40:170.(3) Stöckl D. European specifications for imprecision and inaccuracy compared with US CLIA proficiency-testing criteria. Clin Chem 1995;41:121.(4) Stöckl D, Baadenhuijsen H, Fraser CG, Libeer J-C, Hyltoft Petersen P, Ricós C. Desirable routine analytical goals for quantities assayed in serum. Discussion paper from the members of the external quality assessment (EQA) Working Group A on analytical goals in laboratory medicine. Eur J Clin Chem Clin Biochem 1995;33:157-69.(5) Stöckl, D. The biological approach for specification of analytical quality, a historical overview and the work of the EQAS-organizers Working Group A. Klin Biochem Metab 1996;4:71-3.

Modern aspects of external quality assessment and internal quality control(1) Stöckl D. Consequences of reference method target values in external quality assessment. Klin Biochem Metab 1994;2:55-8.(2) Stöckl D, Thienpont LM. The combined target approach - a way out of the proficiency testing dilemma. Arch Pathol Lab Med 1994;118:775.(3) Stöckl D, Libeer J-C, Reinauer H, Thienpont LM, De Leenheer AP. Accuracy-based assessment of proficiency testing results using serum from single donations: possibilities and limitations. Clin Chem 1996;42:469-70.(4) Ricós C, Baadenhuijsen H, Libeer J-C, Hyltoft Petersen P, Stöckl D, Thienpont LM, Fraser CG. External quality assessment: currently used criteria for evaluating performance in European countries, and criteria for future harmonization. Eur J Clin Chem Clin Biochem 1996;34:159-65.(5) Stöckl, D. Modern Aspects of External Quality Assessment (EQA). Klin Biochem Metab 1996;4:74-7.(6) Libeer J-C, Baadenhuijsen H, Fraser CG, Hyltoft Petersen P, Ricós C, Stöckl D, Thienpont LM. Characterization and classification of external quality assessment schemes (EQA) according to objectives such as evaluation of method and participant bias and standard deviation. Eur J Clin Chem Clin Biochem 1996;34:665-78.(7) Hyltoft Petersen P, Ricós C, Stöckl D, Libeer J-C, Baadenhuijsen H, Fraser CG, Thienpont LM. Proposed guidelines for the internal quality control of analytical results in the medical laboratory. Eur J Clin Chem Clin Biochem 1996;34:983-99.(8) Friedecký B, Kratochvíla J, Budina M, Stöckl D. Comparison of native and processed sera for external quality assessment of sodium and potassium measurements. Klin Biochem Metab 1998;6:109-11.(9) Linko S, Himberg J-J, Thienpont LM, Stöckl D, De Leenheer AP. Assessment of the state-of-the-art trueness and precision of serum total-calcium and glucose measurements in Finnish laboratories - The QSL-Finland-study. Scand J Clin Lab Invest 1998;58:229-40.(10) Thienpont LM, Stockl D, Kratochvila J, Friedecky B, Budina M. Pilot external quality assessment survey for post-market vigilance of in vitro diagnostic medical devices and investigation of trueness of participants' results. Clin Chem Lab Med 2003;41:183-6.(11) Thienpont LM, Stöckl D, Friedecký B, Kratochvíla J, Budina M. Trueness verification in European external quality assessment schemes: time to care about the quality of the samples. Scand J Clin Lab Invest 2003;63:195-202.

Metrology, method comparison, and calibration(1) Stöckl D. Metrology and analysis in laboratory medicine: a criticism from the workbench. Scand J Clin Lab Invest 1996;56:193-7.(2) Stöckl D. Beyond the myths of difference plots [letter]. Ann Clin Biochem 1996;33:575-7.(3) Stöckl D. Difference versus mean plots [reply]. Ann Clin Biochem 1997;34:571.(4) Thienpont LM, Van Nieuwenhove B, Stöckl D, De Leenheer AP. Calibration for isotope dilution mass spectrometry - description of an alternative to the bracketing procedure. J Mass Spectrom 1996;31:1119-25.(5) Hyltoft Petersen P, Stöckl D, Blaabjerg O, Pedersen B, Birkemose E, Thienpont L, Flensted Lassen J, Kjeldsen J. Graphical interpretation of analytical data from comparison of a field method with a reference method by use of difference plots [opinion]. Clin Chem 1997;43:2039-46.(6) Stöckl D, Dewitte K, Thienpont LM. Validity of linear regression in method comparison studies: is it limited by the statistical model or the quality of the analytical input data? Clin Chem 1998;44:2340-6.

(7) Stöckl D, Dewitte K, Fierens C, Thienpont LM. Evaluating clinical accuracy of systems for self-monitoring of blood glucose by error grid analysis. Comment on constructing the “upper A-line”. Diabetes Care 2000;11:1711-2.(8) Petersen PH, Stöckl D, Westgard JO, Sandberg S, Linnet K, Thienpont L. Models for combining random and systematic errors. Assumptions and consequences for different models. Clin Chem Lab Med 2001;39:589-95.(9) Dewitte K, Fierens C, Stöckl D, Thienpont LM. Application of the Bland-Altman plot for interpretation of method-comparison studies: a critical investigation of its practice. Clin Chem 2002;48:799-801;discussion 801-2.

Miscellaneous(1) Thienpont LM, Van Nieuwenhove B, Stöckl D, Reinauer H, De Leenheer AP. Determination of reference method values by isotope dilution-gas chromatography/mass spectrometry: a five years' experience of two European reference laboratories. Eur J Clin Chem Clin Biochem 1996;34:853-60.(2) Hyltoft Petersen P, Fraser CG, Ricós C, Stöckl D, Libeer J-C, Baadenhuijsen H, Thienpont LM. On establishment of common reference intervals in laboratory medicine [letter]. Eur J Clin Chem Clin Biochem 1996;34:515-6.(3) De Brabandere VI, Thienpont LM, Stöckl, De Leenheer AP. 13C-NMR and mass spectral data of steroids with a 17,17-dialkyl-18-nor-13(14)-ene substructure. J Lipid Res 1997:38:190-9.(4) Stöckl D, Franzini C, Kratochvíla J, Middle J, Ricós C, Thienpont LM. Current stage of standardization of measurements of specific polypeptides and proteins discussed in light of steps needed towards a comprehensive measurement system. Eur J Clin Chem Clin Biochem 1997;35:719-32.(5) Stöckl D. Reference materials: organization and realization of quality control at a clinical laboratory (in Russian). Klin Lab Diag 1997(8);47-9.(6) Stöckl D. Modern quality management misunderstood? [letter]. Clin Chem 1998;44:1066-7.(7) Fierens C, Thienpont LM, Stöckl D, De Leenheer AP. Matrix effect in the quantitative analysis of urinary C-peptide by liquid chromatography/mass spectrometry. Rapid Comm Mass Spectrom 2000;14:936-7. (8) Fierens C, Thienpont LM, Stöckl D, De Leenheer AP. Overcoming practical limitations for the application of ultrafiltration in sample preparation for liquid chromatography/mass spectrometry of small proteins. Anal Biochem 2000;285:168-9.(9) Fierens C, Stöckl D, Thienpont LM, De Leenheer AP. A convenient method for the generation of negative and positive electrospray ionization mass spectra of proteins by gas-phase admission of volatile bases and acids via the nebulizing gas. Rapid Commun Mass Spectrom 2001;15:451-3.(10) Dewitte K, Umemoto M, Tani W, Stöckl D, Thienpont LM. Standardization of serum-ionized magnesium to the reference system for serum total magnesium. Trace Elem Electrolytes 2002;19:169-70.(11) Dewitte K, Dhondt A, Lameire N, Stöckl D, Thienpont LM. The ionized fraction of serum total magnesium in hemodialysis patients: is it really lower than in healthy subjects? Clin Nephrol. 2002;58:205-10.