Iodinated contrast agents such as Omnipaque do not interfere with ADAMTS13 activity testing, according to new research.
This finding validates the continued use of fluorescence-based assays for thrombotic thrombocytopenic purpura (TTP) diagnosis, even in patients who have recently received contrast for imaging procedures.
ADAMTS13 activity testing is essential in diagnosing TTP, a rare, life-threatening blood disorder marked by severe enzyme deficiency—defined as less than 10% activity. While other substances are known to interfere with laboratory assays, iodinated contrast had been a suspected source of error.
Addressing this concern, researchers from Vanderbilt University Medical Center and Mayo Clinic tested whether Omnipaque (iohexol), a commonly used contrast agent in CT imaging, interferes with ADAMTS13 measurements in a fluorescence resonance energy transfer (FRET)–based assay. Specifically, they evaluated the ATS-13 ADAMTS13 Activity Assay 2.0 (Immucor), which utilizes FRET technology. Plasma samples were spiked with various Omnipaque concentrations, including up to three times the expected peak level. Some samples were also exposed to light over time to simulate typical lab conditions.
The research team evaluated the excitation, emission, and absorbance spectrum of Omnipaque alone and spiked in patient plasma with known ADAMTS13 activity. They also tested ADAMTS13 activity on an abnormal control sample previously observed to display elevated baseline relative fluorescent units (RFUs). No spectral overlap or fluorescent interference was observed. The fluorophore used in the assay has excitation and emission wavelengths of 492 and 518 nm, while iodine absorbs light at much lower wavelengths.
"No atypical fluorescent peaks were observed on any sample (Omnipaque alone or spiked in plasma) between 250 and 700 nm. There was no difference in the mean ADAMTS13 activity among the various concentrations of plasma spiked with Omnipaque or plasma spiked with saline," reported Jeremy W. Jacobs, MD, MHS, of Vanderbilt University Medical Center, and colleagues.
Activity in Omnipaque-spiked samples was consistent with that of control samples. Even at 16.20 mg/mL, activity remained within the assay's expected variation. One sample at 3.60 mg/mL showed a 2.95% higher result, which was statistically significant but not clinically relevant, as it approximated the typical assay variation (1 SD ± 2%).
Notably, all contrast-spiked samples displayed normal reaction kinetics, with increasing relative fluorescence units over time, unlike samples known to exhibit assay interference.
The results indicated that Omnipaque neither inhibits ADAMTS13 enzyme function nor affects fluorophore detection. Time-dependent effects, such as light exposure or sample degradation, also had no impact on assay performance.
Given that contrast-enhanced imaging is frequently used in the clinical evaluation of suspected TTP, these findings suggest that diagnostic blood draws for ADAMTS13 activity testing do not need to be delayed after contrast administration. The authors specifically stated that "ADAMTS13 activity testing for patients with possible thrombotic thrombocytopenic purpura using this FRET-based assay should not be delayed due to iodinated contrast administration."
Although only Omnipaque was tested, the researchers noted that its low protein binding and rapid excretion may suggest similar findings for other iodinated contrast agents. Nevertheless, the study did not evaluate alternative contrast media.
Ultimately, Omnipaque did not interfere with ADAMTS13 activity measurement, even at high concentrations or with prolonged light exposure. These results support timely TTP testing without concern for contrast-related assay interference.
The authors reported no conflicts of interest.
