Allergy: Evaluation of 16 years (2007-2022) results of the shared external quality assessment program in Belgium, Finland, Portugal and The Netherlands.

OBJECTIVES: This paper evaluates 16 year results of the Allergy EQA program shared by EQA organisers in Belgium, Finland, Portugal, and The Netherlands. METHODS: The performance of Thermo Fisher and Siemens user groups (in terms of concordance between both groups, between laboratory CV, prevalence of clinically significant errors) and suitability of samples (stability and validity of dilution of patient samples) are evaluated using data of 192 samples in the EQA programs from 2007 to 2022. Measurands covered are total IgE, screens and mixes, specific IgE extracts and allergen components. RESULTS: There is perfect (53 %), acceptable (40 %) and poor (6 %) concordance between both method groups. In case of poor concordance the best fit with clinical data is seen for Thermo Fisher (56 %) and Siemens (26 %) respectively. The between laboratory CV evolves from 7.8 to 6.6 % (Thermo Fisher) and 7.3 to 7.7 % (Siemens). The prevalence of blunders by individual laboratories is stable for Siemens (0.4 %) and drops from 0.4 to 0.2 % for Thermo Fisher users. For IgE, the between year CV of the mean of both user groups is 1 %, and a fifteen-fold dilution of a patient sample has an impact of 2 and 4 % on the recovery of Thermo Fisher and Siemens user groups. CONCLUSIONS: The analytical performance of Thermo Fisher is slightly better than that of Siemens users but the clinical impact of this difference is limited. Stability of the sample and the low impact of dilution on the recovery of measurands demonstrates the suitability for purpose of the EQA program.

Evaluation of the ARKRAY HA-8190V instrument for HbA1c.

OBJECTIVES: Hemoglobin A1c (HbA1c) is a valuable parameter in the monitoring of diabetic patients and increasingly in diagnosis of diabetes. Manufacturers continuously optimize instruments, currently the main focus is to achieve faster turnaround times. It is important that performance specifications remain of high enough standard, which is evaluated in this study for the new ARKRAY HA-8190V instrument. METHODS: The Clinical and Laboratory Standards Institute (CLSI) protocols EP-5, EP-9 and EP-10 were applied to investigate imprecision, bias and linearity. In addition potential interferences, performance in External Quality Assessment (EQA) and performance against the HA-8180V instrument in 220 clinical samples was evaluated. RESULTS: The HA-8190V demonstrates a CV of ≤0.8% in IFCC SI units (≤0.6% National Glycohemoglobin Standardization Program [NGSP]) at 34 and 102 mmol/mol levels (5.3 and 11.5% NGSP) and a bias of -0.1 mmol/mol (-0.01% NGSP) at a concentration of 50 mmol/mol (6.7% NGSP), but with a significant slope as compared to target values. This results in a bias of -1.0 and 0.9 mmol/mol (-2.0 and 0.9% NGSP) at the 30 and 70 mmol/mol (4.9 and 8.6% NGSP) concentration level. Simulation of participation in the IFCC certification programme results in a Silver score (bias -0.1 mmol/mol, CV 1.1%). Interference in the presence of the most important Hb variants (AS, AC, AE, AD) and elevated HbA2 and HbF concentrations is less than 3 mmol/mol (0.3% NGSP) at a concentration of 50 mmol/mol (6.7% NGSP). CONCLUSIONS: Analytical performance of the HA-8190V is very good, especially with respect to precision and HbA1c quantification in the presence of the most common Hb variants.

Optimizing hepcidin measurement with a proficiency test framework and standardization improvement.

Objectives: Hepcidin measurement advances insights in pathophysiology, diagnosis, and treatment of iron disorders, but requires analytically sound and standardized measurement procedures (MPs). Recent development of a two-level secondary reference material (sRM) for hepcidin assays allows worldwide standardization. However, no proficiency testing (PT) schemes to ensure external quality assurance (EQA) exist and the absence of a high calibrator in the sRM set precludes optimal standardization. Methods: We developed a pilot PT together with the Dutch EQA organization Stichting Kwaliteitsbewaking Medische Laboratoriumdiagnostiek (SKML) that included 16 international hepcidin MPs. The design included 12 human serum samples that allowed us to evaluate accuracy, linearity, precision and standardization potential. We manufactured, value-assigned, and validated a high-level calibrator in a similar manner to the existing low- and middle-level sRM. Results: The pilot PT confirmed logistical feasibility of an annual scheme. Most MPs demonstrated linearity (R2>0.99) and precision (duplicate CV>12.2%), although the need for EQA was shown by large variability in accuracy. The high-level calibrator proved effective, reducing the inter-assay CV from 42.0% (unstandardized) to 14.0%, compared to 17.6% with the two-leveled set. The calibrator passed international homogeneity criteria and was assigned a value of 9.07±0.24 nmol/L. Conclusions: We established a framework for future PT to enable laboratory accreditation, which is essential to ensure quality of hepcidin measurement and its use in patient care. Additionally, we showed optimized standardization is possible by extending the current sRM with a third high calibrator, although international implementation of the sRM is a prerequisite for its success.

Performance of laboratory tests used to measure blood phenylalanine for the monitoring of patients with phenylketonuria.

Analysis of blood phenylalanine is central to the monitoring of patients with phenylketonuria (PKU) and age-related phenylalanine target treatment-ranges (0-12 years; 120-360 µmol/L, and >12 years; 120-600 µmol/L) are recommended in order to prevent adverse neurological outcomes. These target treatment-ranges are based upon plasma phenylalanine concentrations. However, patients are routinely monitored using dried bloodspot (DBS) specimens due to the convenience of collection. Significant differences exist between phenylalanine concentrations in plasma and DBS, with phenylalanine concentrations in DBS specimens analyzed by flow-injection analysis tandem mass spectrometry reported to be 18% to 28% lower than paired plasma concentrations analyzed using ion-exchange chromatography. DBS specimens with phenylalanine concentrations of 360 and 600 µmol/L, at the critical upper-target treatment-range thresholds would be plasma equivalents of 461 and 768 µmol/L, respectively, when a reported difference of 28% is taken into account. Furthermore, analytical test imprecision and bias in conjunction with pre-analytical factors such as volume and quality of blood applied to filter paper collection devices to produce DBS specimens affect the final test results. Reporting of inaccurate patient results when comparing DBS results to target treatment-ranges based on plasma concentrations, together with inter-laboratory imprecision could have a significant impact on patient management resulting in inappropriate dietary change and potentially adverse patient outcomes. This review is intended to provide perspective on the issues related to the measurement of phenylalanine in blood specimens and to provide direction for the future needs of PKU patients to ensure reliable monitoring of metabolic control using the target treatment-ranges.

Provisional standardization of hepcidin assays: creating a traceability chain with a primary reference material, candidate reference method and a commutable secondary reference material.

Background Hepcidin concentrations measured by various methods differ considerably, complicating interpretation. Here, a previously identified plasma-based candidate secondary reference material (csRM) was modified into a serum-based two-leveled sRM. We validated its functionality to increase the equivalence between methods for international standardization. Methods We applied technical procedures developed by the International Consortium for Harmonization of Clinical Laboratory Results. The sRM, consisting of lyophilized serum with cryolyoprotectant, appeared commutable among nine different measurement procedures using 16 native human serum samples in a first round robin (RR1). Harmonization potential of the sRM was simulated in RR1 and evaluated in practice in RR2 among 11 measurement procedures using three native human plasma samples. Comprehensive purity analysis of a candidate primary RM (cpRM) was performed by state of the art procedures. The sRM was value assigned with an isotope dilution mass spectrometry-based candidate reference method calibrated using the certified pRM. Results The inter-assay CV without harmonization was 42.1% and 52.8% in RR1 and RR2, respectively. In RR1, simulation of harmonization with sRM resulted in an inter-assay CV of 11.0%, whereas in RR2 calibration with the material resulted in an inter-assay CV of 19.1%. Both the sRM and pRM passed international homogeneity criteria and showed long-term stability. We assigned values to the low (0.95±0.11 nmol/L) and middle concentration (3.75±0.17 nmol/L) calibrators of the sRM. Conclusions Standardization of hepcidin is possible with our sRM, which value is assigned by a pRM. We propose the implementation of this material as an international calibrator for hepcidin.

Expressing analytical performance from multi-sample evaluation in laboratory EQA.

BACKGROUND: To provide its participants with an external quality assessment system (EQAS) that can be used to check trueness, the Dutch EQAS organizer, Organization for Quality Assessment of Laboratory Diagnostics (SKML), has innovated its general chemistry scheme over the last decade by introducing fresh frozen commutable samples whose values were assigned by Joint Committee for Traceability in Laboratory Medicine (JCTLM)-listed reference laboratories using reference methods where possible. Here we present some important innovations in our feedback reports that allow participants to judge whether their trueness and imprecision meet predefined analytical performance specifications. METHODS: Sigma metrics are used to calculate performance indicators named 'sigma values'. Tolerance intervals are based on both Total Error allowable (TEa) according to biological variation data and state of the art (SA) in line with the European Federation of Clinical Chemistry and Laboratory Medicine (EFLM) Milan consensus. RESULTS: The existing SKML feedback reports that express trueness as the agreement between the regression line through the results of the last 12 months and the values obtained from reference laboratories and calculate imprecision from the residuals of the regression line are now enriched with sigma values calculated from the degree to which the combination of trueness and imprecision are within tolerance limits. The information and its conclusion to a simple two-point scoring system are also graphically represented in addition to the existing difference plot. CONCLUSIONS: By adding sigma metrics-based performance evaluation in relation to both TEa and SA tolerance intervals to its EQAS schemes, SKML provides its participants with a powerful and actionable check on accuracy.

Toward Worldwide Hepcidin Assay Harmonization: Identification of a Commutable Secondary Reference Material.

BACKGROUND: Absolute plasma hepcidin concentrations measured by various procedures differ substantially, complicating interpretation of results and rendering reference intervals method dependent. We investigated the degree of equivalence achievable by harmonization and the identification of a commutable secondary reference material to accomplish this goal. METHODS: We applied technical procedures to achieve harmonization developed by the Consortium for Harmonization of Clinical Laboratory Results. Eleven plasma hepcidin measurement procedures (5 mass spectrometry based and 6 immunochemical based) quantified native individual plasma samples (n = 32) and native plasma pools (n = 8) to assess analytical performance and current and achievable equivalence. In addition, 8 types of candidate reference materials (3 concentrations each, n = 24) were assessed for their suitability, most notably in terms of commutability, to serve as secondary reference material. RESULTS: Absolute hepcidin values and reproducibility (intrameasurement procedure CVs 2.9%-8.7%) differed substantially between measurement procedures, but all were linear and correlated well. The current equivalence (intermeasurement procedure CV 28.6%) between the methods was mainly attributable to differences in calibration and could thus be improved by harmonization with a common calibrator. Linear regression analysis and standardized residuals showed that a candidate reference material consisting of native lyophilized plasma with cryolyoprotectant was commutable for all measurement procedures. Mathematically simulated harmonization with this calibrator resulted in a maximum achievable equivalence of 7.7%. CONCLUSIONS: The secondary reference material identified in this study has the potential to substantially improve equivalence between hepcidin measurement procedures and contributes to the establishment of a traceability chain that will ultimately allow standardization of hepcidin measurement results.

A multicenter comparison of whole blood vitamin B6 assays.

BACKGROUND: The aim of this study was to compare different analytical methods that are currently in use in the Netherlands for the measurement of whole blood vitamin B6. METHODS: This method comparison study consisted of two separate parts. (1) Four laboratories participated in a pilot study in which the commercial Chromsystems and INstruchemie method, and a laboratory developed liquid chromatography-tandem mass spectrometry (LC-MS/MS) method and HPLC method were compared. Sixty-nine frozen whole blood samples and six lyophilized whole blood samples were used for comparison. (2) In the nationwide part of the study, 49 laboratories participated in the analysis of three identical sets of two whole blood samples of which one set was freshly analyzed, one set was analyzed after storage at -20 °C and one set was analyzed after lyophilization. RESULTS: In both parts of the study, the HPLC and LC-MS/MS methods showed equivalent results for all sample types tested. The Chromsystems method showed a positive bias of 45% (pilot study) and 30% (nationwide study) towards the LC-MS/MS method when fresh or frozen samples were used. The measurement of lyophilized samples showed no differences between the methods. The results of the INstruchemie method were inconclusive due to the low number of participants. CONCLUSIONS: The different analytical methods for measuring vitamin B6 produce different results when whole blood patient samples are measured. The recognition of a reference method or the development of suitable reference materials and quality control materials might serve as a first step towards improved standardization or harmonization of the whole blood vitamin B6 assay.

Two novel haemoglobin variants that affect haemoglobin A1c measurement by ion-exchange chromatography.

BACKGROUND: Haemoglobin (Hb) variants are well-known factors interfering with accurate HbA1c testing. This report describes two novel Hb variants leading to inappropriate quantification of HbA1c by ion-exchange chromatography. METHODS: Glycated forms of novel Hb variants were recognised in the blood of two patients with diabetes mellitus screened by HbA1c ion-exchange chromatography. Dedicated high-resolution cation-exchange chromatography and subsequent DNA sequencing revealed the exact nature of the variants. Other common techniques for quantifying HbA1c were applied on both samples and haematological parameters were determined to judge possible pathology associated with the novel Hb variants. RESULTS: A fraction of 15% of abnormal Hb was observed in a 37-year-old female. DNA sequencing revealed a heterozygous mutation in the α1-globin gene, resulting in a leucine-to-phenylalanine amino-acid substitution (HBA1: c.301C>T, p.Leu101Phe). We named this variant Hb Weesp. The other novel variant, Hb Haelen, presented as a 40% fraction in a 63-year-old male and resulted from a heterozygous amino acid substitution in the ß-globin gene (HBB: c.335T>C, p.Val112Gly). The presence of both Hb variants resulted in aberrant separation of the Hb components, leading to an inadequate quantification of HbA1c. CONCLUSIONS: Close examination of HbA1c chromatograms revealed two novel, clinically silent Hb variants that interfere with HbA1c quantification. Healthcare providers need to be aware of the potential of such Hb variants when interpreting HbA1c results.

Measurement of Hba(1C) in patients with chronic renal failure.

BACKGROUND: Carbamylated hemoglobin (carbHb) is reported to interfere with measurement and interpretation of HbA(1c) in diabetic patients with chronic renal failure (CRF). There is also concern that HbA1c may give low results in these patients due to shortened erythrocyte survival. METHODS: We evaluated the effect of carbHb on HbA(1c) measurements and compared HbA(1c) with glycated albumin (GA) in patients with and without renal disease to test if CRF causes clinically significant bias in HbA(1c) results by using 11 assay methods. Subjects included those with and without renal failure and diabetes. Each subject's estimated glomerular filtration rate (eGFR) was used to determine the presence and degree of the renal disease. A multiple regression model was used to determine if the relationship between HbA(1c) results obtained from each test method and the comparative method was significantly (p<0.05) affected by eGFR. These methods were further evaluated for clinical significance by using the difference between the eGRF quartiles of >7% at 6 or 9% HbA(1c). The relationship between HbA(1c) and glycated albumin (GA) in patients with and without renal failure was also compared. RESULTS: Some methods showed small but statistically significant effects of eGFR; none of these differences were clinically significant. If GA is assumed to better reflect glycemic control, then HbA(1c) was approximately 1.5% HbA(1c) lower in patients with renal failure. CONCLUSIONS: Although most methods can measure HbA(1c) accurately in patients with renal failure, healthcare providers must interpret these test results cautiously in these patients due to the propensity for shortened erythrocyte survival in renal failure.

Measurements of neonatal bilirubin and albumin concentrations: a need for improvement and quality control.

Accurate and precise bilirubin and albumin measurements are essential for proper management of jaundiced neonates. Data hereon are lacking for Dutch laboratories. We aimed to determine variability of measurements of bilirubin and albumin concentrations typical for (preterm) neonates. Aqueous, human serum albumin-based samples with different concentrations of bilirubin (100, 200, 300, 400, and 500 µmol/L) and albumin (0, 10, 15, 20, 25, and 30 g/L) were sent to laboratories of all Dutch neonatal intensive care units (n = 10). Bilirubin and albumin recoveries of the specimens were measured using locally available routine analytical methods. The mean, standard deviation, and coefficients of variations (CV) were calculated per sample. Bilirubin concentrations were underestimated in the absence of albumin (maximal CV 26.0%). When the albumin concentration was 10 or 20 g/L, the bilirubin concentrations of the samples were overestimated (maximal CV 14.1% and 9.2%, respectively). Variability increased with higher weighed-in bilirubin concentrations. Measured albumin levels were ~10% lower than albumin levels of manufactured samples. Bilirubin concentration did not influence albumin measurements. The maximal CV was 6.8%. In conclusion, interlaboratory variability of bilirubin and albumin measurements is high. Recalibration and introduction of a specific quality assessment scheme for neonatal samples is recommended to ensure exchangeability of bilirubin and albumin measurements among laboratories and to control the observed large variability.

Prime time for enzymatic creatinine methods in pediatrics.

BACKGROUND: The literature regarding the nonspecificity of applications of the Jaffe (alkaline picrate) reaction for creatinine is generally outdated. We conducted a specificity study to update the nonspecificity information for current Jaffe and enzymatic creatinine assays. METHODS: Two serum pools with creatinine concentrations within the pediatric reference interval were spiked with albumin, IgG, unconjugated bilirubin, adult hemoglobin (Hb A), and fetal hemoglobin (Hb F) to produce 1 unspiked and 5 spiked samples per pool. The 35 laboratories participating in the survey used a total of 14 method-analyzer combinations. Measurements were performed in triplicate in a single run in accordance with manufacturer instructions. Absolute differences in creatinine concentration between spiked and unspiked samples were calculated per laboratory. Mixed ANOVA was used to quantify the interferent-related CV component for the Jaffe and enzymatic methods. RESULTS: The interference by bilirubin and Hb A on serum creatinine measurements was <10% for most of the Jaffe and enzymatic methods. Obvious interference was observed among the Jaffe methods in samples spiked with Hb F, albumin, and IgG, but not among the enzymatic methods. The within-laboratory interferent-related CVs for the Jaffe method-analyzer combinations ranged from 8.0%-27% at a creatinine concentration of 40.4 micromol/L (0.46 mg/dL) and from 5.4%-15% at 73.4 micromol/L (0.83 mg/dL). Enzymatic methods had within-laboratory interferent-related CVs of <4% at both concentrations. CONCLUSIONS: Albumin, IgG, and Hb F interfered with Jaffe creatinine assays, leading to inaccuracies in estimated glomerular filtration rates that are clinically important, especially in children and neonates. Because protein error and Hb F interference do not occur with any of the enzymatic methods tested, we conclude that enzymatic creatinine methods are preferred for evaluation of kidney function in pediatric cases.

Quality specifications for the determination of copper, zinc, and selenium in human serum or plasma: evaluation of an approach based on biological and analytical variation.

BACKGROUND: Trace element external quality assessment schemes monitor laboratory performance and provide a stimulus for improvement in accuracy. However, monitoring of participant performance varies according to the scheme and can lead to conflicting conclusions. METHODS: Quality specifications based on biological intra- and interindividual variability were calculated and compared to those currently used by various trace element external quality assessment schemes for plasma or serum copper, zinc, and selenium concentrations. For this purpose, we evaluated results reported by participating laboratories in different schemes, at key concentrations, using z scores. RESULTS: Minimal quality specifications developed from the biological intra- and interindividual variability were, for Cu, +/-0.84 micromol/L or 12% of the assigned target concentration, whichever is greater; for Zn, +/-1.20 micromol/L or 15% of the assigned target concentration, whichever is greater; and for Se, +/-0.072 micromol/L or 12% of the assigned target concentration, whichever is greater. Reported performance of the participating laboratories depended on analyte, concentration, and the selected quality specification. In addition, the most commonly used methods for the determination of Cu, Zn, and Se may give different results. CONCLUSIONS: The proposed minimal quality specifications based on biological variation are generally slightly less stringent than those currently in use, although they do not drastically change the performance evaluation in the different schemes. These specifications are a first step in the harmonization of practices among the schemes and remain to be evaluated.

Effects of hemoglobin (Hb) E and HbD traits on measurements of glycated Hb (HbA1c) by 23 methods.

BACKGROUND: Glycohemoglobin (GHB), reported as hemoglobin (Hb) A(1c), is a marker of long-term glycemic control in patients with diabetes and is directly related to risk for diabetic complications. HbE and HbD are the second and fourth most common Hb variants worldwide. We investigated the accuracy of HbA(1c) measurement in the presence of HbE and/or HbD traits. METHODS: We evaluated 23 HbA(1c) methods; 9 were immunoassay methods, 10 were ion-exchange HPLC methods, and 4 were capillary electrophoresis, affinity chromatography, or enzymatic methods. An overall test of coincidence of 2 least-squares linear regression lines was performed to determine whether the presence of HbE or HbD traits caused a statistically significant difference from HbAA results relative to the boronate affinity HPLC comparative method. Deming regression analysis was performed to determine whether the presence of these traits produced a clinically significant effect on HbA(1c) results with the use of +/-10% relative bias at 6% and 9% HbA(1c) as evaluation limits. RESULTS: Statistically significant differences were found in more than half of the methods tested. Only 22% and 13% showed clinically significant interference for HbE and HbD traits, respectively. CONCLUSIONS: Some current HbA(1c) methods show clinically significant interferences with samples containing HbE or HbD traits. To avoid reporting of inaccurate results, ion-exchange chromatograms must be carefully examined to identify possible interference from these Hb variants. For some methods, manufacturers' instructions do not provide adequate information for making correct decisions about reporting results.

Effects of 7 hemoglobin variants on the measurement of glycohemoglobin by 14 analytical methods.

BACKGROUND: Hemoglobin variants (Hb(VAR)) are not uncommon in the Korean population, with Hb G-Coushatta and Hb Queens being the 2 most common Hb(VAR). Hb G-Coushatta is also the most common Hb(VAR) in Chinese people from the Silk Road region, as well as in some North American Indian tribes. However, data are scarce on the effect of these Hb(VAR) on the different methods used for analyzing HbA(1c). METHODS: Specimens from 24 individuals with 7 Hb(VAR) (Hb G-Coushatta, Hb Queens, Hb G-Hsi-Tsou, Hb Ube-4, Hb G-Waimanalo, Hb Inglewood, and Hb Bologna-St.Orsola) were collected and tested using the International Federation of Clinical Chemistry primary reference method as well as 14 routine HbA(1c) assay methods. RESULTS: Hb G-Coushatta showed a clinically significant effect on the measured HbA(1c), particularly when analysis was performed with ion-exchange HPLC methods with short elution times. This interference could be resolved by measuring the HbA(1c) using other methods such as HPLC with a long elution time, immunoassay, boronate affinity chromatography, and enzymatic assay. Hb Queens showed a clinically significant difference, defined as a >10% deviation from regression lines, in results from the 2 HPLC methods but not in the other methods. The remaining 5 rare Hb(VAR) showed different HbA(1c) results in the different assays. CONCLUSION: Hb G-Coushatta, Hb Queens, and other rare Hb(VAR) can interfere with glycohemoglobin assays, including ion-exchange HPLC methods with short elution times, but the interference can be resolved using other unaffected methods. It is important to identify these Hb(VAR) through a careful inspection of the chromatograms and apply other noninterfering methods for accurate measurements of the HbA(1c).

Global standardization of glycated hemoglobin measurement: the position of the IFCC Working Group.

The measurement of glycated hemoglobin is central in the monitoring of glycemic control in patients with diabetes. There are at least 30 different laboratory assays commercially available to measure the proportion of HbA1c in blood. In 1995 the IFCC established a Working Group (IFCC WG-HbA1c) to achieve international standardization of HbA1c measurement. The main achievements can be summarized as follows: a) a reference measurement procedure has been established with purified primary calibrators; b) a network of reference laboratories has been developed worldwide; and c) work has begun on implementation of traceability to the IFCC reference system. The IFCC WG-HbA1c recognizes the recommendation of the IFCC-IUPAC Committee on Nomenclature, Properties and Units that the analyte measured by the IFCC reference measurement procedure has been defined as betaN1-deoxyfructosyl-hemoglobin and that the recommended measurement units are mmol/mol. The IFCC WG-HbA1c recommends maintaining the use of the name HbA1c in clinical practice.

Measurement of urinary porphyrins and porphyrin precursors in Dutch hospital laboratories: a review of quality control over 5 years.

BACKGROUND: We evaluated a quality control scheme for the measurement of urinary uroporphyrin, coproporphyrin, total urinary porphyrins and precursors of urinary porphyrins, delta-aminolevulinic acid and porphobilinogen that was performed in The Netherlands during a period of 5 years. METHODS: Six quality control samples were distributed each year to the participating laboratories. Mean concentrations and the corresponding coefficients of variation were calculated. RESULTS: Coefficients of variation varied widely and were very high in the concentration ranges that can be found in patients with low-grade porphyria. CONCLUSION: Commutable calibrators are needed to improve the laboratory diagnosis of porphyria.