Alternative calibration strategies for the clinical laboratory: application to nortriptyline therapeutic drug monitoring

Comparison between a single- and a multi-point calibration method using LC-MS/MS for measurement of 5-fluorouracil in human plasma

When quantifying therapeutic drugs using LC-MS/MS instrumentation in clinical laboratories, batch-mode analysis with a calibration curve consisting of 6-10 concentrations for each analyte is the most widely used approach. However, this is an inefficient use of this technology since it increases cost, delays result availability and precludes random instrument access. Various alternative methods to reduce the calibrator use and improve efficiency without compromising analytical quality have been investigated, and a single-point calibration has been reported to be the simplest, least expensive and the quickest approach. This study compares a single and a multi-point calibration method using LC-MS/MS with 5-fluorouracil (5-FU) as a model drug. The method was validated for quantitative analysis of 5-FU over a concentration range of 0.05-50 mg/L. Patients undergoing cancer treatment with intravenous 5-FU had plasma 5-FU concentrations measured, and their dose adjusted in real time based on the calculated area under the time-concentration curve (AUC). Subsequently, a single point calibration method using a concentration at 0.5 mg/L was compared to the multi-point calibration method in terms of accuracy and precision. A Bland-Altman bias plot and a Passing-Bablok regression analysis showed a good agreement between the two methods (mean difference = -1.87 %, slope = 1.002, respectively) when comparing patient plasma 5-FU concentrations. The calibration method did not impact the AUC results nor the decision on 5-FU dose adjustments. Our study demonstrated that a single point calibration method produced analytically and clinically comparable results to those produced by a multi-point method when quantifying 5-FU and is feasible to be used clinically.

A Review of Drug Abuse, Misuse, and Related Laboratory Challenges

Various definitions can be considered for drugs and substance abuse. According to the National Institute on Abuse, the use of an over-the-counter drug in a different way than that prescribed to experience or arouse emotion is a simple form of drug abuse. The World Health Organization (WHO) also defines drug abuse as the persistent or sporadic use of drugs that are incompatible or unrelated to acceptable medical practice. With the increasing non-therapeutic use of prescription drugs, serious related consequences have also increased. Therefore, there is a need to know more precisely about the types of substances and drug abuse, which is the most important part of diagnosis and recognizing the tests that cause false positive and negative results. The purpose of this review article is to collect and summarize the most important and more common types of drugs of abuse and review the drugs that cause false results in screening tests. In addition, the most common detection methods of the drug will be reviewed and the advantages and drawbacks of each method will be discussed. In this article, we aimed to point out all the facts about the emerging problems in drug abuse, the methods of screening, and the possible false results in addition to troubleshooting strategies.

From fundamentals in calibration to modern methodologies: A tutorial for small molecules quantification in liquid chromatography-mass spectrometry bioanalysis

Over the last two decades, liquid chromatography coupled to mass-spectrometry (LC‒MS) has become the gold standard to perform qualitative and quantitative analyses of small molecules. When quantitative analysis is developed, an analyst usually refers to international guidelines for analytical method validation. In this context, the design of calibration curves plays a key role in providing accurate results. During recent years and along with instrumental advances, strategies to build calibration curves have dramatically evolved, introducing innovative approaches to improve quantitative precision and throughput. For example, when a labeled standard is available to be spiked directly into the study sample, the concentration of the unlabeled analog can be easily determined using the isotopic pattern deconvolution or the internal calibration approach, eliminating the need for multipoint calibration curves. This tutorial aims to synthetize the advances in LC‒MS quantitative analysis for small molecules in complex matrices, going from fundamental aspects in calibration to modern methodologies and applications. Different work schemes for calibration depending on the sample characteristics (analyte and matrix nature) are distinguished and discussed. Finally, this tutorial outlines the importance of having international guidelines for analytical method validation that agree with the advances in calibration strategies and analytical instrumentation.

Calibration Practices in Clinical Mass Spectrometry: Review and Recommendations

Background

Calibration is a critical component for the reliability, accuracy, and precision of mass spectrometry measurements. Optimal practice in the construction, evaluation, and implementation of a new calibration curve is often underappreciated. This systematic review examined how calibration practices are applied to liquid chromatography-tandem mass spectrometry measurement procedures.

Methods

The electronic database PubMed was searched from the date of database inception to April 1, 2022. The search terms used were "calibration," "mass spectrometry," and "regression." Twenty-one articles were identified and included in this review, following evaluation of the titles, abstracts, full text, and reference lists of the search results.

Results

The use of matrix-matched calibrators and stable isotope-labeled internal standards helps to mitigate the impact of matrix effects. A higher number of calibration standards or replicate measurements improves the mapping of the detector response and hence the accuracy and precision of the regression model. Constructing a calibration curve with each analytical batch recharacterizes the instrument detector but does not reduce the actual variability. The analytical response and measurand concentrations should be considered when constructing a calibration curve, along with subsequent use of quality controls to confirm assay performance. It is important to assess the linearity of the calibration curve by using actual experimental data and appropriate statistics. The heteroscedasticity of the calibration data should be investigated, and appropriate weighting should be applied during regression modeling.

Conclusions

This review provides an outline and guidance for optimal calibration practices in clinical mass spectrometry laboratories.

System Performance Monitoring in Clinical Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)

Quality assurance (QA) activities enable continuous improvement through ongoing post-implementation monitoring to identify, evaluate, and correct problems. QA for clinical liquid chromatography tandem mass spectrometry (LC-MS/MS) assays should include specific components that address the unique aspects of these methods. This chapter briefly describes approaches for clinical LC-MS/MS system performance monitoring using batch and peak review metrics, largely following CLSI-C62A guidance. Though routine checks ensure the quality of results reported for each run, there is also a need to evaluate metrics between runs over time. Post-implementation performance monitoring of LC-MS/MS methods is typically focused on calibration curves, retention times, peak intensities, and ion ratios.

Isotopic Distribution Calibration for Mass Spectrometry

Mass spectrometry (MS) is widely used in science and industry. It allows accurate, specific, sensitive, and reproducible detection and quantification of a huge range of analytes. Across MS applications, quantification by MS has grown most dramatically, with >50 million experiments/year in the USA alone. However, quantification performance varies between instruments, compounds, different samples, and within- and across runs, necessitating normalization with analyte-similar internal standards (IS) and use of IS-corrected multipoint external calibration curves for each analyte, a complicated and resource-intensive approach, which is particularly ill-suited for multi-analyte measurements. We have developed an internal calibration method that utilizes the natural isotope distribution of an IS for a given analyte to provide internal multipoint calibration. Multiple isotope distribution calibrators for different targets in the same sample facilitate multiplex quantification, while the emerging random-access automated MS platforms should also greatly benefit from this approach. Finally, isotope distribution calibration allows mathematical correction for suboptimal experimental conditions. This might also enable quantification of hitherto difficult, or impossible to quantify, targets, if the distribution is adjusted in silico to mimic the analyte. The approach works well for high resolution, accurate mass MS for analytes with at least a modest-sized isotopic envelope. As shown herein, the approach can also be applied to lower molecular weight analytes, but the reduction in calibration points does reduce quantification performance.

A 24-Hour Extended Calibration Strategy for Quantitating Tacrolimus Concentrations by Liquid Chromatography-Tandem Mass Spectrometry

BACKGROUND

Tacrolimus has a low therapeutic index requiring strict control of whole blood concentrations. Although random access immunoassay platforms exist that rapidly provide quantitative values for tacrolimus, LC-MS/MS may provide more accurate quantitation. However, batch testing in many LC-MS/MS assays is not efficient, particularly when testing patients suspected of having tacrolimus toxicity. Extending calibration curve stability beyond the traditionally accepted single batch may facilitate improved turnaround time and reduce testing costs. A 24-h extended calibration of LC-MS/MS tacrolimus was designed and validated to reduce calibrator usage, improve turnaround time, and provide a more efficient workflow for urgent requests.

METHODS

Patient samples included in the study were extracted and assayed with coextracted calibrators and quality control in real time. The same patient samples were extracted again 24 h later without coextracted calibrators. The data acquired from the second patient sample extraction was applied to the original calibration curve acquired 24 h prior and compared to the data for the same samples coextracted with calibrators, creating a value set utilizing extended curve stability.

RESULTS

A linear regression compared the results using the extended curve to the results of the coextracted acquisitions. This yielded a strong correlation between the 2 data populations, with a slope of 1.0061 and a correlation coefficient of >0.95. The average bias between original patient values and patient values 24 h later was 3.4% across all patient samples.

CONCLUSIONS

Patient tacrolimus values were comparable when extracted within 24 h of calibration versus values coextracted with calibrators. Demonstrating comparability within 24 h of calibration allows the laboratory to provide rapid turnaround time for urgent samples without the need for an entirely new calibration curve.

Calibrating from Within: Multipoint Internal Calibration of a Quantitative Mass Spectrometric Assay of Serum Methotrexate

BACKGROUND

Clinical LC-MS/MS assays traditionally require that samples be run in batches with calibration curves in each batch. This approach is inefficient and presents a barrier to random access analysis. We developed an alternative approach called multipoint internal calibration (MPIC) that eliminated the need for batch-mode analysis.

METHODS

The new approach used 4 variants of 13C-labeled methotrexate (0.026-10.3 µM) as an internal calibration curve within each sample. One site carried out a comprehensive validation, which included an evaluation of interferences and matrix effects, lower limit of quantification (LLOQ), and 20-day precision. Three sites evaluated assay precision and linearity. MPIC was also compared with traditional LC-MS/MS and an immunoassay.

RESULTS

Recovery of spiked analyte was 93%-102%. The LLOQ was validated to be 0.017 µM. Total variability, determined in a 20-day experiment, was 11.5%CV. In a 5-day variability study performed at each site, total imprecision was 3.4 to 16.8%CV. Linearity was validated throughout the calibrator range (r2 > 0.995, slopes = 0.996-1.01). In comparing 40 samples run in each laboratory, the median interlaboratory imprecision was 6.55%CV. MPIC quantification was comparable to both traditional LC-MS/MS and immunoassay (r2 = 0.96-0.98, slopes = 1.04-1.06). Bland-Altman analysis of all comparisons showed biases rarely exceeding 20% when MTX concentrations were >0.4 µM.

CONCLUSION

The MPIC method for serum methotrexate quantification was validated in a multisite proof-of-concept study and represents a big step toward random-access LC-MS/MS analysis, which could change the paradigm of mass spectrometry in the clinical laboratory.

Advances in bioanalytical techniques to measure steroid hormones in serum

Steroid hormones are measured clinically to determine if a patient has a pathological process occurring in the adrenal gland, or other hormone responsive organs. They are very similar in structure making them analytically challenging to measure. Additionally, these hormones have vast concentration differences in human serum adding to the measurement complexity. GC–MS was the gold standard methodology used to measure steroid hormones clinically, followed by radioimmunoassay, but that was replaced by immunoassay due to ease of use. LC–MS/MS has now become a popular alternative owing to simplified sample preparation than for GC–MS and increased specificity and sensitivity over immunoassay. This review will discuss these methodologies and some new developments that could simplify and improve steroid hormone analysis in serum.

Measurement of Estradiol in Human Serum by LC-MS/MS Using a Novel Estrogen-Specific Derivatization Reagent

A liquid chromatography-tandem mass spectrometry (LC-MS/MS) method is described that employs a novel derivatization reagent for the measurement of serum estradiol (E2), with simultaneous analysis of underivatized testosterone (T) and dihydrotestosterone (DHT). The main advantage of the new derivatization reagent 1,2-dimethylimidazole-5-sulfonyl chloride is its analyte-specific fragmentation that enables monitoring of confirmatory mass transitions with high sensitivity. The reaction mixture can be analyzed without additional purification steps using a 9.5 min gradient run, and sensitive detection is achieved with a triple quadrupole mass spectrometer using atmospheric pressure photoionization. Method validation was performed with human serum samples, including a comparison with a standard LC-MS/MS method using 120 samples from a clinical study, and analysis of certified E2 serum reference materials BCR-576, BCR-577, and BCR-578. The lower limits of quantification for E2, T, and DHT were 0.5 pg/mL, 25 pg/mL, and 0.10 ng/mL, respectively, from a 200-μL sample. Validation results indicated good accuracy and agreement with established, conventional LC-MS/MS assays, demonstrating suitability for analysis of samples containing E2 in the low pg/mL range, such as serum from men, children, and postmenopausal women.

Diagnostic Frequency Ratios Are Insufficient to Measure Laboratory Precision with The Bethesda System for Reporting Thyroid Cytopathology

OBJECTIVE

Diagnostic frequency ratios such as the atypia of undetermined significance (AUS):malignant ratio are touted to be useful for laboratory precision benchmarking. We therefore sought to examine their reproducibility and usefulness at a tertiary hospital.

METHODS

We reviewed thyroid fine-needle aspirates (FNA) submitted to our institution from outside laboratories and evaluated the ability of diagnostic frequency ratios to capture the complexity of The Bethesda System for Reporting Thyroid Cytopathology (TBSRTC). Specifically, we evaluated the ability of the AUS:malignant ratio to describe the frequencies of the other TBSRTC diagnoses.

RESULTS

A total of 2,784 cases from 19 laboratories were included. The use of the AUS category varied the most. There was insufficient reflection of the non-AUS nonmalignant TBSRTC diagnostic frequencies in our analysis, and these results do not appear to arise from observer variability in the outside laboratories.

CONCLUSION

Diagnostic frequency ratios are not reproducible in our experience and fail to describe the other TBSRTC categories. As such, they are unlikely to prove sufficient for benchmarking laboratory precision with TBSRTC.

Some unnecessary or inadequate common practices in regulated LC–MS bioanalysis

The global bioanalytical community increasingly craves scientifically sound practices and guidance where the rationale is given for each requirement. To this end, it is critical to first evaluate all the existing practices and requirements based on scientific findings and critical thinking. Here we are challenging several important common practices in regulated LC–MS bioanalysis, from the requirement of at least six different calibration concentrations, no extrapolation, use of blank and zero standard in each batch, selection of quality controls, to the way matrix effect and dilution integrity are being validated. Both the reasons why these common practices are unnecessary or inadequate and the potential solutions are presented.

Deproteination of whole blood for LC-MS/MS using paramagnetic micro-particles

OBJECTIVES

Liquid chromatography tandem mass spectrometry has become increasingly popular in clinical laboratories over the last decade due to the inherent sensitivity and specificity of the technology. Nevertheless, full automation and hence application in routine laboratories is still hampered by laborious and difficult-to-automate sample pre-treatment protocols. Functionalized paramagnetic micro-particles could simplify sample pre-treatment and ease automation. We evaluated the applicability of a pre-commercial, straightforward paramagnetic micro-particle based kit for the lysis and deproteination of whole blood for the LC-MS/MS analysis of everolimus and compared the performance to our routine protein precipitation method.

DESIGN AND METHODS

Samples were prepared for LC-MS/MS everolimus analysis on an Acquity UPLC chromatographic system coupled to a TQD mass spectrometer (both Waters Ltd.) using a pre-commercial MagSi-TDMprep kit and a routine protein precipitation respectively. Both pre-treatment methods were validated for imprecision, accuracy, linearity, limit of quantification, matrix effect, recovery and process efficiency. A method comparison (n=63) between both pre-treatment methods was performed.

RESULTS

Imprecision and bias were within pre-defined criteria (15%) for both pre-treatment methods. Both methods were linear from 1.2 to 14.8μg/L with a limit of quantification of 0.6μg/L. Process efficiency was on average 65% for protein precipitation pre-treatment and was substantially higher for the MagSi-TDMprep method (85%). A Passing-Bablok regression showed no significant difference between the two pre-treatment methods.

CONCLUSION

For everolimus in whole blood, the MagSi-TDMprep sample pre-treatment was applicable and comparable to protein precipitation for LC-MS/MS with the possible advantage of easier automation.

Reproducibility of the Johns Hopkins Hospital template for urologic cytology samples

INTRODUCTION

Cytologic screening for urothelial carcinoma is fraught with low sensitivity, a high indeterminate rate, and until recently, poor standardization of terminology. The Johns Hopkins Hospital John K. Frost Cytopathology Laboratory has recently developed and published a template for reporting urine cytopathology; herein, we evaluate its interobserver reproducibility.

MATERIALS AND METHODS

Two sets of 100 cases each were deidentified; each set was reviewed by 5 of 10 observers in a randomized order at the direction of computerized data collection software that tracked observation time as well as observer classification of the atypia-no atypia, atypia (AUC-US), or atypia suggestive of high-grade urothelial carcinoma (AUC-H). Specific morphologic features were also recorded. Cases were grouped into low-, intermediate-, and high-agreement based on the number of observers who made the assessment. The findings were correlated against clinical outcomes.

RESULTS

High agreement among observers about the presence or absence of high-grade features was possible in approximately two-thirds of indeterminate urine cases. Time and order did not factor significantly into observer propensity for identifying atypical features or favoring either AUC-US or AUC-H, and cases with high agreement about the presence of high-grade features were more likely to have a malignant follow-up. Furthermore, AUC-H diagnoses based on 2 or more high-grade features had a significantly higher malignancy risk than AUC-US diagnoses did.

CONCLUSIONS

AUC-H is a valid diagnostic category with specific, reproducibly identified features that portend a higher risk of malignancy than the findings of AUC-US.

Challenges in implementing clinical liquid chromatography-tandem mass spectrometry methods--the light at the end of the tunnel

The use of liquid chromatography-tandem mass spectrometry (LC-MS/MS) in the clinical setting is a relatively new application. One of the significant barriers hampering the transition of LC-MS/MS from the research lab into a clinical setting is the uncertainty of how to successfully develop and validate a method that meets guidelines for clinical applications. Here, we have taken this seemingly overwhelming process and broken it into five general stages for consideration: assessing the clinical validity of a new LC-MS/MS assay, determination of feasibility, assay development, assay validation and post-implementation monitoring. Although various publications are available and serve as resources for determining development processes and acceptability criteria for specific LC-MS/MS assays, many of them are general recommendations or are specific to research applications that may not translate either practically or clinically. In this perspective special feature article, a resource is compiled that describes key differences between LC-MS/MS methods for research use versus clinical use. In addition, the challenges facing the expanding role of this technique in the clinical setting are discussed, including instrumentation/automation challenges, potential regulation of laboratory developed tests by the US Food and Drug Administration and standardization and harmonization of MS methods through the use of traceable materials and availability of guidance documents.