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Estimation of precision and inaccuracy for serum magnesium determination on the basis of interlaboratory comparison data Accreditation ISO 15189


Magnesium Research. Volume 21, Number 1, 51-7, march 2008, original article

DOI : 10.1684/mrh.2008.0121

Summary  

Author(s) : Silvia Izquierdo Álvarez, Alicia Boudet García, Santiago Otero Martínez, María Dolores Fernández González, Joaquín González Revaldería, Ángel García De Jalón Comet, Jesús Fernando Escanero Marcén , Servicio de Bioquímica Clínica, Hospital Universitario Miguel Servet, Zaragoza;, Laboratorio Central, Hospital Clinico Universitario de Santiago, Santiago de Compostela, A Coruña;, Laboratorio de Bioquímica, Hospital Universitario Marqués de Valdecilla, Santander, Cantabria;, Servicio de Bioquímica, Hospital Universitario de Getafe, Madrid;, Departamento de Farmacología y Fisiología, Facultad de Medicina, Uuniversidad de Zaragoza, Spain.

Summary : Serum Mg is an important biochemical parameter in the context of clinical medicine for monitoring patients and for helping to diagnose some pathologies. The clinical laboratories must offer analytical results of quality in all parameters determined, demonstrating this way the laboratory “skill competence”. The aim of this study was to revalidate (ISO 15189 standard) some different colorimetric methods for Mg determination in serum used in clinical and/or biochemical laboratories in four hospitals in Spain, on the basis of results of interlaboratory comparison programmes: Bio-Rad EQAS and external quality control SEQC. Precision and inaccuracy were estimated by analysis of records of an external quality control programme for Mg. The precision and inaccuracy values obtained were both less than 10%, except in one hospital in which the precision was less than 15%. These values of precision and inaccuracy obtained may be considered highly satisfactory taking into account the validation requirement for these ones: less than 10%. These findings demonstrate the effectiveness of the new revalidation methodology for diagnostic methods in medicine, which does not require any disruption of the laboratory’s routine activity and which can be used even if the method in question has not been validated previously. It is also suggested that the ideas and requirements of ISO 15189 should be followed by the research laboratories.

Keywords : magnesium, accreditation, validation, interlaboratory comparison programmes, ISO 15189, external quality control, precision, inaccuracy

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ARTICLE

Auteur(s) : Silvia Izquierdo Álvarez1, Alicia Boudet García1, Santiago Otero Martínez2, María Dolores Fernández González3, Joaquín González Revaldería4, Ángel García De Jalón Comet1, Jesús Fernando Escanero Marcén5

1Servicio de Bioquímica Clínica, Hospital Universitario Miguel Servet, Zaragoza;
2Laboratorio Central, Hospital Clinico Universitario de Santiago, Santiago de Compostela, A Coruña;
3Laboratorio de Bioquímica, Hospital Universitario Marqués de Valdecilla, Santander, Cantabria;
4Servicio de Bioquímica, Hospital Universitario de Getafe, Madrid;
5Departamento de Farmacología y Fisiología, Facultad de Medicina, Uuniversidad de Zaragoza, Spain

Magnesium (Mg) is the most abundant intracellular divalent cation present in human organism and participates in numerous physiological processes [1, 2]. The serum Mg determination is an important biochemical parameter in the context of clinical medicine for monitoring patients and for helping to diagnose some pathologies. Magnesium deficiency is known to be linked with cardiovascular alterations and several renal, gastrointestinal, neurological and muscular disorders [2, 3]. Each clinical and/or biochemical laboratory has responsibility for demonstrating its competence and therefore must obtain results of good (required) quality.

The ISO (International Standard Organization) 15189 standard specifies the quality management system requirements, in particular to medical laboratories and states:

  • “the laboratory shall use only validated procedures for confirming that the examination procedures are suitable for the intended use”;
  • “the validations shall be as extensive as are necessary to meet the needs in the given application or field of application”;
  • “procedures need to be periodically revalidated in view of changing conditions and technical advances” [4, 5].

Precision and inaccuracy are two of the key validation parameters for any instrumental method in the determination of trace elements and Mg. These parameters can be estimated on the basis of interlaboratory comparison programmes, or the external quality control programmes in which participation is required for ISO 15189 accreditation [1, 6, 7].

The magnesium research field is more complex (more variety of specimens and different methodologies) than clinical laboratories (where automatized — colorimetric — methods to determine Mg in serum are usually used). The standards of quality control in Mg research are determined by the researchers themselves and by the magazines where they publish their work.

According to these facts, the aims of the present study were: 1) to estimate the precision and inaccuracy of different colorimetric methods for Mg determination in serum, used in different hospitals, on the basis of the results of participation in interlaboratory comparison programmes and 2) to establish a common methodology in the estimation of precision and inaccuracy for the Mg determination methods used in scientific laboratories.

Material and methods

The results of participation in the external quality control (Quality Control Interlaboratory Programmes: Bio-Rad EQAS and external quality control SEQC) for serum Mg determination with colorimetric methods were analysed during one year (2006). The colorimetric methods used were:
  • the Mg method based on the modified xylidyl blue reaction, described by Mann and Yoe [8]. Analytical parameters: analytical range: 1.6 to 5.0 mg/dL for serum, C.V. < 5%;
  • another colorimetric method with chlorophosphonazo III [9]. Analytical parameters: precision and accuracy < 5%;
  • the last colorimetric method with calmagite, LX 20 apparatus, auto-analyser with photometric module. Analytical parameters have not been indicated by manufacturers.

All records of participation in these interlaboratory programmes were studied in four different clinical laboratories of hospitals (Santiago, Santander, Getafe and Zaragoza). From these data a series of paired values were obtained: VL, the concentration given by the laboratory, and VR, the target value of the interlaboratory comparison programme. It was necessary to obtain at least ten pairs of values by the same analytical method within the range of concentration considered as usual.

The steps for estimation of precision and accuracy (expressed as inaccuracy: 100 – accuracy), are indicated in a flow chart (figure 1). Inaccuracy is expressed in quantitative terms while accuracy is a qualitative expression. This is the reason that justifies the election of inaccuracy.

The data were grouped in a table 1 and the calculations were performed using Microsoft Excel.

With respect to second aim of this study, a review of all original papers published in Magnesium Research during 2005 and 2006 was carried out. The type of sample, the methodology used for Mg determination and some quality requirements were studied.
Table 1 Summary of data values obtained from records of an intercomparison programme for estimation of precision and inaccuracy.

VR (interlaboratory comparison value)

VL. (laboratory value)

Differences: dn = VLn - VRn

Relative differences (accuracies): An = Dn / VRn

VR1

VL1

d1 = VL1 - VR1

d1 / VR1

VR2

VL2

d2 = VL2 - VR2

d2 / VR2

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

VRn

VLn

dn = VLn - VRn

dn / VRn

Results

1. Precision and inaccuracy of colorimetric methods used in serum Mg determination in four different hospitals of Spain.

Precision and inaccuracy values obtained in all laboratories analysed (four) were less than 10%, except in one hospital in which the precision was less than 15% (table 2).

For all ranges of concentration established, the higher value of precision was 12.52% (Zaragoza-Aragón, Spain) and the laboratory of Santiago de Compostela (A Coruña, Spain) obtained the best value (1.41%). Although all colorimetric methods analyzed gave acceptable values of precision (< 15%), the chlorophosphorazo III one used in this last laboratory is best adjusted to this methodology.

In general, these results should be considered as acceptable according to the validation requirement, because the precision and inaccuracy were less than 10%.

2. Methodology for the estimation of some basic parameters of validation in laboratories dedicated to Mg research.

The results found are presented in table 3.

The precedent table shows the Mg determination methods and samples used in papers published in Magnesium Research during 2005 and 2006. With respect to the methods, 21 groups [15-35] used spectrophotometric methods versus only five groups [10-14] that reported colorimetric ones. Moreover, one group worked with the method of Thuvasethakul & Wajjwalku [36], another with the ICAP one [3] and, finally, one of them quotes the Mg2+ selective electrode [37]. In relation to the samples, the different specimens studied can be observed. Compared with the clinical laboratories, the researchers prefered to use spectrophotometric methods for Mg determination in a great variety of samples.
Table 2 Summary of values of precision and inaccuracy by the four laboratories analysed in four hospitals in Spain.

Santander (Cantabria)

Getafe (Madrid)

Zaragoza (Aragón)

Santiago de Compostela (A Coruña)

Precision

6.91%

6.66%

12.52%

1.41%

Inaccuracy

-3.49%

-3.87%

-3.46%

-1.27%

Method

  • Colorimetric
  • Xylidyl Blue/ADIVA
  • 1650 Siemens Chemistry systems


Colorimetric

  • Colorimetric
  • Calmagite
  • Beckman/LX-20 apparatus


  • Colorimetric
  • Chlorophosphonazo III/COBAS INTEGRA 400


Microrange

1.80-4.50 mg/100 mL

0.50-2.00 nmol/L

1.10-3.15 mg/100 mL

1.10-4.50 mg/100 mL

Intercomparison programme

BIO RAD Laboratories

CONTROL SEQC

BIO RAD Laboratories

BIO RAD Laboratories


Table 3 Magnesium determination methods and samples used in papers published in Magnesium Research during 2005 and 2006.

Reference

Type of analytical method

Sample

Günther [15]

Null point method AAS

Erythrocytes

Dabrowski [10]

Colorimetric test xylidine blue

Serum

Chaudhary, et al. [36]

Method of Thuvasethakul &Wajjwalku

Blood

Nielsen, et al. [3]

ICAP-Ash Atom Com 1140

Serum-urine-feces

Dunicz-Sokolowska, et al. [16]

AAS method

Hair

Karakiewicz, et al. [17]

Flame AAS

Serum

Billyard, et al. [18]

Flame AAS

Plasma

Dunicz-Sokolowska, et al. [19]

In flame

Hair

Van Orden, et al. [20]

Flame Atomic Perkin Elmer 306.

Plasma

Grases, et al. [21]

Atomic emission spectrometry

Urine 24 hours

Pasternak, et al. [22]

Spectrophotometric methods

Blood

Pasternak, et al. [23]

Spectrophotometric methods

Blood

Ebel and Günther [24]

AAS

Erythrocytes

Katsumata, et al. [25]

AAS

Serum

Nemoto, et al. [26]

AAS

Feces and urine

Dunicz–Sokolowska, et al. [27]

Flame

Hair

Mousain-Bosc, et al. [11]

Colorimetric (chlorophosphonazo III)

Serum and intra-erythrocyte

Mousain-Bosc, et al. [12]

Colorimetric (chlorophosphonazo III)

Serum and intra-erythrocyte

Coudray, et al. [28]

Perkin Elmer

Blood

Sabbagh, et al. [13]

Xylidyl blue complexometric

Plasma

Sabbagh, et al. [14]

Xylidyl blue complexometric

Plasma

Kozielec, et al. [37]

Ion selective magnesium electrode

Blood

Pasternak, et al. [29]

Spectrophotometric methods

Blood

Korycinska, et al. [30]

Spectrophotometric methods

Blood

Iskra, et al. [31]

AAS

Serum

Katsumata, et al. [32]

AAS

Feces

Matsuzaki, et al. [33]

AAS

Feces

Feillet-Coudray, et al. [34]

AAS

Chiu, et al. [35]

Spectrophotometric methods

Drinking water

Discussion

The results of precision and inaccuracy of colorimetric methods used in theclinical laboratories studied was obtained using information from interlaboratory comparison programmes. This methodology was implemented for the first time by our group and, consequently, the results reported can not be compared with previous ones. However, the precision and inaccuracy values obtained for all laboratories analysed could be considered in the same range as those reached by a classical method (experimental estimation of precision: reproducibility, repetitively and accuracy of the method studied) of validation. This agreement shows the effectiveness of this new revalidation methodology for clinical methods, much used for diagnostics in medicine.

At the moment, ISO 15189 is one recommendation that brings progress in the obligations of clinical and/or biochemical laboratories, specially the requirement of validation/revalidation of analytical methods. Now, consequently, these laboratories can see ISO 15189 as a guide to achieve accreditation and to begin to practice some kind of methodology in order to validate/revalidate their analytical methods.

In relation to Mg research laboratories, assuming the use of an important variety of methodologies (spectrophotometry being the main one) in different samples, they could participate in external quality programmes or generate a specific methodology, as reported in this paper. None of the papers referred in table 3 [3, 10, 11, 13-37] reported quality aspects and values of parameters of validation with respect to the methods used for Mg determination. In the near future, the values of precision and inaccuracy estimated in the validation/revalidation of any one method of Mg determination could be reported in all publications, specifically in scientific papers, not only in those concerning clinical laboratories.

Conclusion

The methodology that uses information on interlaboratory comparison programmes to validate some colorimetric methods for Mg determination in serum is useful, economical, fast and recommendable. It avoids the need to disrupt routine work to carry out validation trials, it does not require previous validation trials, and it establishes the same validation parameters as those reached by classical methodology.

This methodology could be applied to laboratories dedicated to Mg research. Parameters of quality control such as precision and inaccuracy could be reported in scientific papers independently of the method used for Mg determination.

References

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31 Iskra M, Baralkiewicz D, Majewski W, Piorunska-Stolzmann M. Serum magnesium, copper and zinc concentration changes in lower limb ischemia and postoperative treatment. Magnes Res 2005; 18: 261-7.

32 Katsumata SI, Matsuzaki H, Uehara M, Suzuki K. Effect of dietary magnesium supplementation on bone loss in rats fed on high phosphorus diet. Magnes Res 2005; 18: 91-6.

33 Matsuzaki H, Katsumata SI, Uehara M, Suzuki K, Nakamura K. Effects of high calcium intake on bone metabolism in magnesium-deficient rats. Magnes Res 2005; 18: 97-102.

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