Quantitative tandem mass spectrometry in the clinical laboratory: Regulation and opportunity for validation of laboratory developed tests

Tandem mass spectrometry is an important analytical tool for clinical laboratories, but tests developed and validated in-house (laboratory developed tests, or LDTs) require special consideration. In late 2022, the forecast for United States (U.S.) federal regulation of LDTs changed unexpectedly when the VALID Act was not passed by the U.S. Congress. This Act would have modified the Food and Drug Administration's (FDA's) role to increase regulatory oversight for LDT providers. In this revised context, we review optimization of quantitative mass spectrometry LDT validation and suggest avenues other than an additional FDA mandate to achieve uniform best practice. Common challenges, logistical barriers, and recommendations for easing the burden of best-quality quantitative mass spectrometry LDT method validation are discussed.

Quantitation of Thyroglobulin in Serum Using SISCAPA and Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)

Accurate measurement of thyrogloblulin (Tg) at low concentrations is essential for recurrence-monitoring in patients who have been treated for papillary and follicular thyroid cancers. The immunoassays commonly employed by clinical laboratories to measure Tg are known to suffer interferences from thyroglobulin autoantibodies (TgAb).We describe a semiautomated stable isotope standards and capture by antipeptide antibodies (SISCAPA®) LC-MS/MS method for the accurate and precise measurement of Tg using 400 uL of serum. Following trypsin digestion of serum proteins in a 96-well plate format, a Tg-specific peptide is captured and concentrated using a monoclonal antibody bound to protein G-coated paramagnetic beads. Eighteen microliters of concentrate are injected into the LC-MS/MS system. Quantitation is performed against a 6-point linear calibration curve prepared in a blank matrix. The assay calibration range is 0.1-10 ng/mL, the range of clinical interest for recurrence detection. Total imprecision in clinical production has been observed to be 13.8% and 6.54% for in-house prepared control materials having Tg concentrations of 0.24 ng/mL and 0.94 ng/mL, respectively. Limit of quantitation was determined to be 0.1 ng/mL.

Quantitation of IgG Subclasses in Serum Using Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)

Serum IgG subclasses (IgGSC) are measured for a number of indications, but the most common are the identification of selective immunodeficiency disease and the diagnosis of IgG4-related disease (IgG4RD). Traditional nephelometric (IN) assays can suffer from two issues impacting the accuracy of the results: (1) hook effect and (2) antibody cross-reactivity between the subclasses. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is not vulnerable to these modes of interference and therefore serves as an excellent and relatively inexpensive means of diagnosing and/or monitoring the relevant clinical conditions.We describe a semiautomated and simple method for the accurate and precise measurement of IgGSC from 20 μL of serum using a liquid chromatography and tandem mass spectrometry (LC-MS/MS) method following digestion of serum proteins in 96-well plate format. Due to the high abundance of the target proteins, no specialized sample preparation (such as solid phase extraction) is required. Twenty microliters are injected to the LC-MS/MS system. Quantitation is performed against a five-point duplicate linear calibration curve prepared in blank matrix. The assay calibration range is 0.38-7.74 g/L for IgG1, 0.24-4.46 g/L for IgG2, 0.038-0.752 g/L for IgG3, 0.025-0.435 g/L for IgG4, and 0.62-15.5 g/L for total IgG. Total IgG is used as an internal quality control marker and is compared to the sum of the four subclass results. Total imprecision in clinical production has been observed to be 5.1-10.6% for in-house prepared control materials having IgGSC mean values in the range of 0.38-8.43 g/L for IgG1, 0.22-3.76 g/L for IgG2, 0.0387-0.721 g/L for Ig3, and 0.0279-1.46 g/L for IgG4. Limit of quantitation (LoQ) was determined to be 0.29 g/L for IgG1, 0.22 g/L for IgG2, 0.019 g/L for IgG3, and 0.0067 g/L for IgG4.

Establishing pre-analytical requirements and maximizing peptide recovery in the analytical phase for mass spectrometric quantification of amyloid-β peptides 1-42 and 1-40 in CSF

OBJECTIVES

Amyloid-β (Aβ) peptides in cerebrospinal fluid (CSF), including Aβ42 (residues 1-42) and Aβ40 (residues 1-40), are utilized as biomarkers in the diagnostic workup of Alzheimer's disease. Careful consideration has been given to the pre-analytical and analytical factors associated with measurement of these peptides via immunoassays; however, far less information is available for mass spectrometric methods. As such, we performed a comprehensive evaluation of pre-analytical and analytical factors specific to Aβ quantification using mass spectrometry.

METHODS

Using our quantitative mass spectrometry assay for Aβ42 and Aβ40 in CSF, we investigated the potential for interference from hemolysate, bilirubin, lipids, and anti-Aβ-antibodies. We also optimized the composition of the calibrator surrogate matrix and Aβ recovery during and after solid phase extraction (SPE).

RESULTS

There was no interreference observed with total protein up to 12 g/L, hemolysate up to 10% (v/v), bilirubin up to 0.5% (v/v), intralipid up to 1% (v/v), or anti-Aβ-antibodies at expected therapeutic concentrations. For hemolysate, bilirubin and lipids, visual CSF contamination thresholds were established. In the analytical phase, Aβ recovery was increased by ∼50% via SPE solvent modifications and by over 150% via modification of the SPE collection plate, which also extended analyte stability in the autosampler.

CONCLUSIONS

Attention to mass spectrometric-specific pre-analytical and analytical considerations improved analytical sensitivity and reproducibility, as well as, established CSF specimen acceptance and rejection criteria for use by the clinical laboratory.

An automated clinical mass spectrometric method for identification and quantification of variant and wild-type amyloid-β 1-40 and 1-42 peptides in CSF

INTRODUCTION

We developed an automated liquid chromatography-tandem mass spectrometry high performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS) method for multiplex quantification of wild-type (wt) amyloid β (Aβ) peptides 1-40 (Aβ40) and 1-42 (Aβ42) and detection of variant Aβ peptides in cerebrospinal fluid.

METHODS

The multiplex Aβ HPLC-MS/MS assay was validated in a clinically accredited laboratory following regulatory guidelines, with Aβ42 calibration assigned to the ERM/IFCC certified reference material; sequence variants were additionally multiplexed into the method.

RESULTS

Sample preparation was fully automated on a liquid handler. The assay quantified wt-Aβ42 and wt-Aβ40 and detected sequence variants, when present, within the Aβ42 sequence.

DISCUSSION

Extension of the HPLC-MS/MS approach for quantification of wt-Aβ42 and wt-Aβ40 to include known sequence variants increases analytical accuracy of the mass spectrometric approach and enables identification of cases of autosomal dominant Alzheimer's disease. Development of an automated workflow and selection of appropriate instrumentation enabled deployment of this method in routine clinical testing.

Comparison of four clinically validated testosterone LC-MS/MS assays: Harmonization is an attainable goal

Introduction

Immunoassays and liquid chromatography-tandem mass spectrometry assays are commonly employed in clinical laboratories for measurement of total testosterone in serum. Results obtained from either of these methodologies compare poorly due to differences in calibration and/or inadvertent detection of interfering substances by the immunoassays. Standardization efforts are underway, but recent studies indicate that accuracy remains an issue.

Methods

This study compares the results from four independently developed and validated LC-MS/MS assays for total testosterone. The calibration for each assay was verified using National Institute of Standards and Technology Standard Reference Material 971.

Results

Initially, one of the four assays had a mean percent difference of +11.44%, compared to the All Method Mean, but following re-verification of all five non-zero calibrator concentrations with the NIST SRM 971, the mean percent difference decreased to -4.88%. Subsequently, the agreement between all four assays showed a mean bias of <5% across the range of all testosterone concentrations (0.13-38.10 nmol/L; 3.7-1098 ng/dL), including at low concentrations of <1 nmol/L (<29 ng/dL).

Conclusions

Excellent agreement between four independently developed LC-MS/MS assays demonstrates that harmonization using standard reference material is attainable. However, as we found in this study, to ensure accurate calibration it is critical to validate the concentrations of new lots of calibrators.