基于生物学变异的质量规范和六西格玛管理方法在临床生化质量控制中的应用研究

目的探讨基于生物学变异的质量规范和六西格玛(6σ)管理方法在临床生化质量控制中的应用研究。方法根据Westgard生物学变异数据库提供的质量规范,对28个临床生化项目计算西格玛(σ)值;允许不精密度[百分变异系数(CV%)表示]的数据是采用本实验室2018年3-9月室内质控数据;百分偏倚(bias%)的数据是采用2018年第二次卫生部临床生化室间质评比对的偏倚数据;根据σ值和功效函数图选择合适的质控规则,以及根据质量目标指数(QGI)查找出不合格项目的主要原因,为实验室全面质量控制管理奠定坚实的基础。结果28个临床生化项目中,有2个、9个和15个项目分别选择了最低、合适和最佳的CV%生物学变异质量规范,Na和Mg未能达到最低目标要求;分别有12个和10个选择了合适和最佳的bias%生物学变异质量规范,Na、Cl、Ca、ALB、TP未能达到最低目标要求;分别有5个、11个和4个选择了最低、合适和最佳的允许总误差(TEa)生物学变异质量规范,GLU、Ca、TP、Na、Cl、Mg、ApoA1、ApoB未能达到6σ目标要求。未能达到6σ目标要求的项目,通过计算QGI值发现需要优先改进精密度的项目占87.5%。结论运用生物学变异不同水平的质量规范和6σ质量管理方法,可以有效地提高临床实验室质量水平,是临床实验室开展质量控制的一项有效的管理工具。     

基于生物学变异的质量规范在临床生化质控方案设计中的应用

目的 探讨基于生物学变异的质量规范在临床生化质控方案设计中的应用.方法 按照Westgard等[7]的报道方法对18个临床生化项目基于生物学变异不同水平质量目标对应的sigma值进行了计算.不精密度(CV％)的数据采用近6个月(2011年5月～10月)室内质控数据.偏倚(Bias)的数据采用该室参加Bio-Rad实验室间质控比对计划(Inter laboratoryQCProgram)中与对等组比较的最近(2011年10月)的偏倚数据.根据sigma值及质控方案性能选择合适的质量目标及质控方案.结果 18个生化项目中,分别有7个、2个和6个项目选择了最佳、期望和最低水平的生物学变异的质量规范.Ca,Cl和Na目前的性能不能满足最低水平生物学变异质量规范的要求,分别选择了CLIA' 88和德国Rilibak质控指南的标准.结论 应用基于生物学变异不同水平的质量规范,可以让临床清楚了解检测过程给实验结果带来的误差最大有多少,更有利于检测结果的临床应用.     

血清游离脂肪酸酶法测定的方法学评价及生物学变异研究

目的评价血清游离脂肪酸(FFA)酶学比色法(简称酶法)的精密度和正确度,并且研究血清FFA个体间和个体内生物学变异。方法采用美国临床和实验室标准化协会(CLSI)EP5-A2和EP15-A文件评价酶法测定血清FFA的精密度和正确度。生物学变异的研究对象为13名健康志愿者,在6周时间内每2周测定1次血清FFA浓度,利用SAS9.1软件中MIXED分析过程计算个体间生物学变异和个体内生物学变异。结果精密度实验样品的血清FFA浓度总均数为0.796 mmol/L,批内、批间、日间和室内不精密度分别为2.15%、3.44%、3.67%和5.46%。测定3个室间质评样品的百分比偏差分别为24.20%、-0.90%、-1.50%。血清FFA个体间和个体内生物学变异分别为8.43%、19.36%。结论酶法测定血清FFA的精密度和正确度能满足相关实验室质量控制要求。血清FFA的个体间和个体内生物学变异可为制定实验室质量规范和临床课题的科研设计提供参考依据。     

上海地区女性卵泡期血清性激素指标长期生物学变异研究

目的 了解上海地区女性卵泡期血清性激素指标长期生物学变异数据,以导出基于生物学变异的性能规范。方法 招募上海交通大学医学院附属国际和平妇幼保健院64名志愿者(20～40岁非妊娠期女性),采集每位志愿者每月月经期第2或3天外周血样本,持续6个月,检测7项血清性激素指标[抗米勒管激素(AMH)、雌二醇(E2)、促卵泡激素(FSH)、黄体生成素(LH)、泌乳素(PRL)、孕酮(Prog)、睾酮(Testo)]水平。根据相关公式导出“最佳”“适当”“最低”3个水平的允许不精密度、允许偏移和允许总误差。结果 与欧洲临床化学和检验医学联合会(EFLM)数据库数据相比,Prog、Testo项目个体内变异(CV_I)和个体间变异(CV_G),FSH项目CV_I与已有数据差异无统计学意义(P>0.05);E2项目CV_I和CV_G均显著高于已有数据(P<0.01);PRL、LH项目CV_I和CV_G,FSH项目CV_G,AMH项目CV...     

生物学变异在患者系列结果改变评价中的应用

临床实验室检验结果的固有分析变异分解为:分析前变异(sP),分析变异(sA),个体内生物学变异(s1).结合统计学原理计算各检验项目的 参考变化值(RCV),然后与实际变化差值比较判断患者一系列的检验结果间是否有显著改变.结合生物学变异探索评价患者一系列检验结果改变的显著性的方法,可以帮助临床正确解读患者的检验结果.临床实验室可以使用RCV和变化差值显著性概率为系列检验结果显著改变的临床监测提供客观的指南和帮助.     

生化室内质控变异系数的分析

目的了解我科常规生化项目的室内质控不精密度水平,找出与行业标准的差距,不断改进,以提高室内质量控制和检验质量水平。方法将21个生化常规项目一年的室内质控实测累积变异系数,分别列出最小CV值、50%分位数CV值、最大CV值,与卫生行业标准规定的《临床生物化学检验常规项目分析质量指标》要求进行比较。结果 TBI、DBI、ALB、AST、ALP、GGT、GLU、CHO、TG、K、Na、Cl、UA、CRE共14项的CV值符合行业标准允许CV值的要求。TP、Ca水平1、2和ALT水平2的最小CV值、50%分位数CV值、最大CV值均大于卫生行业标准允许CV值;LDH、BUN水平1、2和AMY、Mg水平2的50%分位数CV值、最大CV值均大于行业标准允许CV值。结论变异系数反应室内质控的不精密度,依据卫生行业标准WS/T403-2012作室内质控参数设置的参考,有利于室内质量控制的统一和正确评估,有利于提高室内质量控制和检验质量。     

分析变异和个体内生物学变异与检验结果的关系

目的利用生物学变异建立差值显著性评价方法,并用于检验结果评价。方法通过室内质量控制数据获得贝克曼AU68全自动生化仪的累计在控变异系数,结合个体内生物学变异计算肝肾功能和电解质等12项生化项目的参考变化值,并评价慢性肾炎患者连续监测的检验结果。结果仪器的分析变异相对固定,总的分析变异与个体内生物学变异相关。差值显著性计算后慢性肾炎患者治疗3个月时的总蛋白、清蛋白、丙氨酸氨基转移酶和肌酐有显著性改变;治疗后8个月肾功能的指标变化没有显著性。结论临床实验室可以使用差值显著性评价方法对患者检验结果和监测指标的系列结果变化进行评价,帮助临床医生科学解读检验报告。     

已通过ISO 15189认可的临床实验室常规生化指标的性能评价

目的评价已通过ISO 15189认可的临床实验室的常规生化指标的检测性能。方法采集已通过ISO 15189认可的临床实验室2012年第1次全国常规化学室间质量评价(EQA)和室内质量控制(IQC)数据,根据基于生物学变异的质量规范,对K、Na、Cl、Ca等24个指标的EQA和累计在控CV的及格率进行统计;评估各指标的西格玛(σ)水平;计算各指标的实验室间平均CV,与CAP 2012年常规化学调查总结结果进行比较。结果 EQA活动中,平均80%以上的实验室K、Glu、Urea等18个指标满足适当的质量规范,平均80%以上的实验室K、Urea、UA等14个指标满足最佳的质量规范;IQC活动中,平均80%以上的实验室K、P、Glu等16个指标的累积CV满足适当的质量规范,80%以上的实验室TG、T-Bil等6个指标的累积在控CV满足最佳的质量规范;>80%的实验室Urea、UA、T-Bil、ALT和GGT达到3<σ≤6;>80%的实验室TG、D-Bil、CK和Fe达到σ>6;>20%的实验室Na、Cl、Ca、Alb、ALP和Mgσ≤0。各指标实验室间平均CV与CAP调查结果相当。结论通过ISO 15189认可的实验室检测能力基本满足基于生物学变异的适当的质量规范,但个别指标仍需继续改进质控策略,提高检测结果的可靠性和准确性。     

我国34个常规化学检验项目性能规范研究

目的为我国34个常规化学检验项目推荐允许总误差(TEa)、允许不精密度(CV)和允许偏倚(Bias)。方法根据意大利米兰会议最新确定的性能规范设定模式, 基于生物学变异(BV)和基于当前技术水平模式导出性能规范。通过Clinet-EQA软件收集参加国家卫生健康委临床检验中心2019至2021年常规化学和脂类分析EQA活动实验室的数据(包括EQA数据和IQC数据)。对于有BV数据的检验项目, 将各研究项目EQA数据的"百分差值"、IQC数据的"当月在控CV"分别与基于BV导出的3种水平评价标准比较, 计算各年份所有批号的百分差值通过率和CV通过率, 当通过率达到80%, 则该水平的性能规范满足作为该项目推荐性能规范的要求。对于无BV数据的检验项目或者基于BV导出的3种水平性能规范均无法作为推荐标准的检验项目, 则基于当前技术水平来导出推荐性能规范。各检验项目在获得推荐TEa和允许CV后, 利用公式|Bias|≤TEa-z×CV导出推荐允许Bias。结果 34个检验项目推荐的TEa(CV)%结果如下:K4.7(2), Na4(1.5), Cl 4(1.4), Ca5(2), P9.6(3....     

以生物学变异为质量要求评价免疫球蛋白等项目的过程能力

目的探讨如何以生物学变异为质量目标评价免疫球蛋白、补体等项目的过程能力。方法按照室内质量控制的常规方法采用朗道公司2个水平特定蛋白质控品,在4个月时间内每个工作日检测免疫球蛋白A（IgA）等7个项目,计算各项目实际的变异系数（CV）、偏倚、允许总误差（TEa）和σ值。结果以生物学变异确定的最低TEa作为质量目标时,IgA、免疫球蛋白M（IgM）和C反应蛋白（CRP）的σ值〉4.0;补体C3和C4的σ值3.6,IgG的σ值2.5;类风湿因子（RF）的σ值1.5。结论以生物学变异确定的TEa作为质量目标,能较好地了解各检测项目实际CV、偏倚TEa和σ值满足质量要求的程度,评价其分析过程能力的高低。     

Biological variation and reference change values of CA 19-9, CEA, AFP in serum of healthy individuals

OBJECTIVE: The use of tumour markers in diagnosis and monitoring is very common. Tumour marker results vary - preanalytical sources of variation, total random analytical error (CV(a)), and within-subject (intraindividual) normal biological variation. There are not so many studies evaluating the biological variations and reference change values (RCV) of these parameters. The aim of our study was to assess: (i) the average inherent intra- and inter-individual biological variation (CV(i) and CV(g)) for CA 19-9, CEA, AFP in a group of healthy individuals; (ii) the significance of changes in serial results of each marker; and (iii) the index of individuality. MATERIAL AND METHODS: The study group comprised 49 healthy volunteers ranging in age between 18 and 60 years (25 M and 24 F). Four blood samples were obtained from each subject; one at each 14-day interval. Each sample from one individual was assayed in duplicate. CA 19-9, CEA, AFP levels were measured by an immunoluminometric assay on a random-access analyser (Architect i2000; Abbott Diagnostics Division). The intra- (CV(i)) and inter-individual (CV(g)) biological variations were estimated from the data generated. Reference change value (RCV) was calculated. RESULTS: The intra-individual/inter-individual biological variations (CVs) for CA 19-9, CEA, AFP were 27.2/64.24 %, 30.87/37.14 % and 26.67/43.65 %, respectively. The critical differences (RCVs) of CA 19-9, CEA, AFP were 64.71 %, 72.57 % and 62.62 %, respectively (Z = 1.65 for unidirectional changes; p<0.05). CONCLUSIONS: Intra-individual biological variation contributes to the variation in serial results and should therefore be included in the criteria for serum tumour marker assessment.     

Influence of short-term venous stasis on clinical chemistry testing.

Control and standardization of preanalytical variability is a critical factor for achieving accuracy and precision in laboratory testing. Although venous stasis from tourniquet placement during venepuncture should be minimized, as it has been claimed to account for spurious and significant variations for several analytes in plasma, there is controversy surrounding its real impact on laboratory testing. The aim of the present study was the investigation of the influence of short-term venous stasis on routine biochemical testing, by measuring the plasma concentration of 12 common analytes, including proteins, protein-bound substances, enzymes and electrolytes, in plasma specimens collected either without venous stasis or following the application of standardized external pressure of 60 mm Hg using a sphygmomanometer for 1 and 3 min. Although the overall correlation between measures was acceptable, the pattern of change was mostly dependent on the length of stasis, size and protein-binding characteristics of the analytes, achieving clinical significance for albumin, calcium and potassium after 1-min stasis, and alanine aminotransferase, albumin, calcium, chloride, total cholesterol, glucose and potassium after 3-min stasis. Statistically significant differences could be observed in seven (alanine aminotransferase, albumin, calcium, total cholesterol, creatine kinase, iron and potassium) and ten (alanine aminotransferase, albumin, calcium, chloride, total cholesterol, creatine kinase, creatinine, glucose, iron and potassium) out of the 12 analytes tested, after 1- and 3-min venous stasis, respectively. The most clinically significant changes from standard venepuncture, when compared to the current analytical quality specifications for desirable bias, occurred for potassium (1-min stasis, -2.8%; 3-min stasis, -4.8%, both p<0.001), calcium (1-min stasis, +1.6%, p<0.05; 3-min stasis, +3.6%, p<0.001) and albumin (1-min stasis, +3.5%; 3-min stasis, +8.6%, both p<0.001). As most of these effects are dependent on the stasis time during venepuncture and biochemical or physiological characteristics of the analyte, these variations could likely be anticipated, allowing the most appropriate preventive measures to be adopted.     

Influence of hemolysis on routine clinical chemistry testing.

BACKGROUND: Preanalytical factors are the main source of variation in clinical chemistry testing and among the major determinants of preanalytical variability, sample hemolysis can exert a strong influence on result reliability. Hemolytic samples are a rather common and unfavorable occurrence in laboratory practice, as they are often considered unsuitable for routine testing due to biological and analytical interference. However, definitive indications on the analytical and clinical management of hemolyzed specimens are currently lacking. Therefore, the present investigation evaluated the influence of in vitro blood cell lysis on routine clinical chemistry testing. METHODS: Nine aliquots, prepared by serial dilutions of homologous hemolyzed samples collected from 12 different subjects and containing a final concentration of serum hemoglobin ranging from 0 to 20.6 g/L, were tested for the most common clinical chemistry analytes. Lysis was achieved by subjecting whole blood to an overnight freeze-thaw cycle. RESULTS: Hemolysis interference appeared to be approximately linearly dependent on the final concentration of blood-cell lysate in the specimen. This generated a consistent trend towards overestimation of alanine aminotransferase (ALT), aspartate aminotransferase (AST), creatinine, creatine kinase (CK), iron, lactate dehydrogenase (LDH), lipase, magnesium, phosphorus, potassium and urea, whereas mean values of albumin, alkaline phosphatase (ALP), chloride, gamma-glutamyltransferase (GGT), glucose and sodium were substantially decreased. Clinically meaningful variations of AST, chloride, LDH, potassium and sodium were observed in specimens displaying mild or almost undetectable hemolysis by visual inspection (serum hemoglobin < 0.6 g/L). The rather heterogeneous and unpredictable response to hemolysis observed for several parameters prevented the adoption of reliable statistic corrective measures for results on the basis of the degree of hemolysis. CONCLUSION: If hemolysis and blood cell lysis result from an in vitro cause, we suggest that the most convenient corrective solution might be quantification of free hemoglobin, alerting the clinicians and sample recollection.     

Biological variation and reference change values of CA 19‐9, CEA,AFP in serum of healthy individuals

Objective. The use of tumour markers in diagnosis and monitoring is very common. Tumour marker results vary – preanalytical sources of variation, total random analytical error (CV_a), and within‐subject (intraindividual) normal biological variation. There are not so many studies evaluating the biological variations and reference change values (RCV) of these parameters. The aim of our study was to assess: (i) the average inherent intra‐ and inter‐individual biological variation (CV_i and CV_g) for CA 19‐9, CEA, AFP in a group of healthy individuals; (ii) the significance of changes in serial results of each marker; and (iii) the index of individuality. Material and methods. The study group comprised 49 healthy volunteers ranging in age between 18 and 60 years (25 M and 24 F). Four blood samples were obtained from each subject; one at each 14‐day interval. Each sample from one individual was assayed in duplicate. CA 19‐9, CEA, AFP levels were measured by an immunoluminometric assay on a random‐access analyser (Architect i2000; Abbott Diagnostics Division). The intra‐ (CV_i) and inter‐individual (CV_g) biological variations were estimated from the data generated. Reference change value (RCV) was calculated. Results. The intra‐individual/inter‐individual biological variations (CVs) for CA 19‐9, CEA, AFP were 27.2/64.24?%, 30.87/37.14?% and 26.67/43.65?%, respectively. The critical differences (RCVs) of CA 19‐9, CEA, AFP were 64.71?%, 72.57?% and 62.62?%, respectively (Z = 1.65 for unidirectional changes; p<0.05). Conclusions. Intra‐individual biological variation contributes to the variation in serial results and should therefore be included in the criteria for serum tumour marker assessment.     

Influence of the needle bore size used for collecting venous blood samples on routine clinical chemistry testing.

BACKGROUND: Despite remarkable advances in technology and laboratory automation, results of laboratory testing still suffer from a high degree of preanalytical variability. Although there is no definitive evidence, the use of small-gauge needles for venipuncture is usually discouraged to reduce the chance of producing unsuitable specimens. METHODS: The purpose of this investigation was to assess the influence of the needle size used to collect venous blood on the measurement of 14 common analytes, including free hemoglobin, the most representative enzymes, protein-bound substances and electrolytes. Results for venous blood samples collected from 20 fasting voluntary physicians using either a 23- (0.60 mmx19 mm) or 25-gauge-needle (0.50 mmx19 mm) butterfly devices with polyvinyl chloride tubing (1.40 mmx300 mm) were compared with reference specimens collected using a 21-gauge-needle (0.80 mmx19 mm) butterfly device with polyvinyl chloride tubing (1.40 mmx300 mm). RESULTS: All means for paired samples collected using the smaller needles did not differ significantly from the reference specimen by paired Student's t-test analysis. Passing-Bablok regression analysis and Pearson's or Spearman (creatine kinase, aspartate aminotransferase, alanine aminotransferase and chloride) correlation were acceptable for most of the analyses, although a lower correlation coefficient was observed for electrolytes. In addition, when expressed as a percentage of the mean for paired samples, the s(y,x) value exceeded the desirable bias for free hemoglobin, glucose, lactate dehydrogenase, aspartate aminotransferase, sodium, chloride, calcium and magnesium (in samples collected using both 23 G and 25 G needles) and potassium (in samples collected using a 25 G needle). Although Bland-Altman plot analysis and +/-1.96 SD agreement intervals for the set of differences between values was acceptable overall, the bias was rather broad for free hemoglobin and several critical electrolytes (calcium, chloride, potassium, sodium), exceeding the respective limits for desirable bias. CONCLUSIONS: The results of our investigation indicate that 23 G needles, if handled correctly, will not introduce any statistically or clinically significant error to the measurement results compared to a 21 G needle. For the 25 G needle, we observed increased variability for potassium compared to a 23 G needle. Small-bore needles of 25 G or less cannot be universally recommended when collecting venous blood for clinical chemistry testing and should be reserved for selected circumstances, such as in patients with problematical venous accesses and newborns. In such cases, however, the bias introduced by the use of smaller needles should always be taken into consideration when interpreting test results.     

Circadian and seasonal variations of electrolytes in aging humans

The circadian and seasonal variations of a set of routinely determined variables (chloride, sodium, potassium, calcium, inorganic phosphorus, magnesium, creatinine, urea and urate) were documented in young men (mean age ± SD: 24.0 ± 3.9 yr) and in healthy elderly men (75.3 ± 6.6) and women (78.2 ± 9.1). The same urinary variables, except magnesium, were studied in young men. The circadian variability of serum variables was between 2 and 11% except for serum inorganic phosphorus (12–22% according to the group). By contrast, urinary chloride, sodium and potassium revealed large peak-trough differences (55–75%) and the variability of urinary creatinine, urate and urea was also not negligible (20–30%). ANOVA validated seasonal variations for most of the plasma variables and for urinary calcium, phosphorus and uric acid. No age or sex difference in either 24 h means or amplitudes could be observed. These data are of interest for the concept of reference values, for the diagnosis of certain bone and renal disease as well as for chronooptimization in treatment of potential electrolytes deficiency states.     

Depression of serum cholinesterase activity as an indicator for insecticide exposure — consideration of the analytical and biological variations

Interest has recently been shown in the use of serum cholinesterase levels as a marker for chronic organophosphate insecticide exposure. Interpretation of the results must, however, take into account both long-term biological and analytical variations. The intraindividual variation of serum cholinesterase activity for 12 laboratory workers was 4.3%. The variation was much smaller than the interindividual variation of the whole population (20%). Based on the homogeneity of the intraindividual variance, it is possible to show the level of minimal decrease in enzyme activity which constitutes a departure from "normal".     

The impact of pre-analytical variations on biochemical analytes stability: A systematic review

ObjectiveA common problem in clinical laboratories is maintaining the stability of analytes during pre‐analytical processes. The aim of this study was to systematically summarize the results of a set of studies about the biochemical analytes stability.MethodsA literature search was performed on the Advanced search field of PubMed using the keywords: “(stability) AND (analytes OR laboratory analytes OR laboratory tests OR biochemical analytes OR biochemical tests OR biochemical laboratory tests).” A total of 56 entries were obtained. After applying the selection criteria, 20 articles were included in the study.ResultsIn the 20 included references, up to 123 different analytes were assessed. The 34 analytes in order of the most frequently studied analytes were evaluated: Alanine aminotransferase, aspartate aminotransferase, potassium, triglyceride, alkaline phosphatase, creatinine, total cholesterol, albumin, lactate dehydrogenase, sodium, calcium, γ‐glutamyltransferase, total bilirubin, urea, creatine kinase, inorganic phosphate, total protein, uric acid, amylase, chloride, high‐density lipoprotein, magnesium, glucose, C‐reactive protein, bicarbonate, ferritin, iron, lipase, transferrin, cobalamin, cortisol, folate, free thyroxine, and thyroid‐stimulating hormone. Stable test results could be varied between 2 hours and 1 week according to the type of samples and/or type of blood collection tubes on a basic classification set as refrigerated or room temperature.ConclusionsBiochemical analytes stability could be improved if the best pre‐analytical approaches are used.     

Impact of blood collection devices on clinical chemistry assays

Blood collection devices interact with blood to alter blood composition, serum, or plasma fractions and in some cases adversely affect laboratory tests. Vascular access devices may release coating substances and exert shear forces that lyse cells. Blood-dissolving tube additives can affect blood constituent stability and analytical systems. Blood tube stoppers, stopper lubricants, tube walls, surfactants, clot activators, and separator gels may add materials, adsorb blood components, or interact with protein and cellular components. Thus, collection devices can be a major source of preanalytical error in laboratory testing. Device manufacturers, laboratory test vendors, and clinical laboratory personnel must understand these interactions as potential sources of error during preanalytical laboratory testing. Although the effects of endogenous blood substances have received attention, the effects of exogenous substances on assay results have not been well described. This review will identify sources of exogenous substances in blood specimens and propose methods to minimize their impact on clinical chemistry assays.     

Anticoagulant-induced preanalytical errors in ionized calcium determination on blood

When anticoagulated blood is necessary for ionized calcium (Ca2+) measurements especially in urgent circumstances, the type (sodium or 'calcium-titrated' heparinate) as well as the form (aqueous or dry) of anticoagulant induce preanalytical errors. To quantify these modifications Ca2+ was measured in three aqueous solutions and in three serum pools in different 'sampling' conditions. Incomplete syringe filling and specimen volume/syringe nominal volume ratio effects were tested. Syringes were rinsed (i) with saline to yield 'pure' dilution effect ('solution-dilution'); (ii) with sodium heparinate to study binding; (iii) with calcium-titrated heparinate to evaluate 'calcium-distortion'. All types of errors increased when syringes were not filled to their nominal volume, especially on small-sized specimens. Detailed tables provide percentage error values for all sampling conditions. Thus, 'solution-dilution' can reach -5%; binding is always important (-15 to -50%). 'Calcium-distortion' is minimal, around 1.25 mmol/l Ca2+, but can reach -7% for high Ca2+ and +10% for low Ca2+.