An Opioid Hiding in Plain Sight: Loperamide-Induced False-Positive Fentanyl and Buprenorphine Immunoassay Results

Advances in fentanyl testing

Fentanyl is a synthetic opioid that was approved by the FDA in the late 1960s. In the decades since, non-prescription use of fentanyl, its analogs, and structurally unrelated novel synthetic opioids (NSO) has become a worsening public health crisis. There is a clear need for accessible testing for these substances in biological specimens and in apprehended drugs. Immunoassays for fentanyl in urine are available but their performance is restricted to facilities that hold moderate complexity laboratory licenses. Immunoassays for other matrices such as oral fluid (OF), blood, and meconium have been developed but are not widely available. Point of care tests (POCT), such as lateral flow immunoassays or fentanyl test strips (FTS), are widely available but not approved by the FDA for clinical use. All immunoassays are vulnerable to false positive and false negative results. Immunoassays may or may not be able to detect fentanyl analogs and NSOs. Mass spectrometry (MS) can accurately and reliably measure fentanyl and its major metabolite norfentanyl in urine and oral fluid. MS is available at reference laboratories and large hospitals. Liquid chromatography paired with tandem mass spectrometry (LC-MS/MS) is the most widely used method and has outstanding specificity and sensitivity for fentanyl and norfentanyl. When compared to immunoassays, MS is more expensive, requires more technical skill, and takes longer to result. Newer mass spectrometry methods can measure fentanyl analogs and NSO. Both mass spectrometry assays and immunoassays [in the form of fentanyl test strips (FTS)] have potential use in harm reduction programs.

Evaluating the Performance of the Roche FEN2 Fentanyl Immunoassay and Its Clinical Implementation: The Role of LDT-Based Mass Spectrometry Testing

Introduction

While laboratory-developed tests (LDTs) using liquid chromatography tandem mass spectrometry (LC-MS/MS) are widely employed to support the development of FDA-cleared drug immunoassays, their significance in the clinical implementation and evaluation of such assays is often overlooked. This paper reports on the important role of LC-MS/MS LDTs in demonstrating improved performance of the Roche FEN2 fentanyl immunoassay compared with the Thermo DRI fentanyl immunoassay.

Methods

The FEN2 assay was implemented according to the manufacturer's instructions and its performance was compared to the existing DRI assay using LC-MS/MS as a reference. Clinical sensitivity and specificity were determined using 250 consecutive random patient specimens. Spiking experiments were conducted to determine cross-reactivity with 31 fentanyl analogs. Select DRI false-positive samples were analyzed by the FEN2 assay via time-of-flight mass spectrometry method (LC-QTOF).

Results

The FEN2 assay showed improved clinical sensitivity compared to the DRI (98% vs 61%) in 250 consecutive patient samples due to its ability to detect norfentanyl. It also showed better clinical specificity by correctly classifying select DRI false-positive results. Upon implementation in clinical practice, the FEN2 resulted in a higher screening positivity rate than the DRI (17.3% vs 13.3%) and a greater LC-MS/MS confirmation rate of immunoassay-positive samples (96.8% vs 88.8%, respectively).

Conclusion

The use of LC-MS/MS LDTs demonstrated that the FEN2 assay has greater clinical sensitivity and is less prone to false-positives than the DRI assay. These findings support the use of FEN2 in routine clinical practice and emphasize the role of mass spectrometry-based LDTs in clinical toxicology testing.

Impact of the loss of Laboratory Developed Mass Spectrometry testing at a major academic medical center

Background

Our laboratory historically performed immunosuppressant and definitive opioid testing in-house as laboratory developed (LDT) mass spectrometry-based tests. However, staffing constraints and supply chain challenges associated with the COVID-19 pandemic forced us to refer this testing to a national reference laboratory. The VALID Act could impose onerous requirements for laboratories to develop LDTs. To explore the potential effect of these additional regulatory hurdles, we used the loss of our own LDT tests to assess the impact on patient care and hospital budgets.

Methods

Laboratory information systems data and historical data associated with test costs were used to calculate turnaround times and financial impact.

Results

Referral testing has extended the reporting of immunosuppressant results by an average of approximately one day and up to two days at the 95th percentile. We estimate that discontinuing in-house opioid testing has cost our health system over half a million dollars in the year since testing was discontinued.

Conclusions

Barriers that discourage laboratories from developing in-house testing, particularly in the absence of FDA-cleared alternatives, can be expected to have a detrimental effect on patient care and hospital finances.