Abstract: TH-PO1080

Interference of Tiopronin with Urine Cystine Measurement Is Assay-Dependent

Session Information

Category: Mineral Disease

  • 1204 Mineral Disease: Nephrolithiasis

Authors

  • Giesen, Callen D, Mayo Clinic, Rochester, Minnesota, United States
  • Chirackal, Robin Sunny, Mayo Clinic, Rochester, Minnesota, United States
  • Brady, Clayton, None, Fairport, New York, United States
  • Voskoboev, Nick, Mayo Clinic, Rochester, Minnesota, United States
  • Flanagan, Ryan M, Mayo Clinic, Rochester, Minnesota, United States
  • Lieske, John C., Mayo Clinic, Rochester, Minnesota, United States
Background

Patients with cystinuria often suffer frequent stone events, require multiple surgical interventions, and even develop CKD. When conservative measures fail (fluids and alkali), thiol drugs including tiopronin that reduce cystine to create tiopronin-cysteine dimers (and thus increase cystine solubility) are the next option. Urine cystine is often measured using a cyanide-nitroprusside colorimetric assay. Quantitation by liquid chromatography tandem mass spectrometry (LC-MSMS) is specific but not readily available. The present study compared urine cystine measurement by both methods to determine the relative effect of tiopronin.

Methods

Waste urine samples (n=16) from the Mayo Clinic Renal Testing Laboratory were spiked to expected therapeutic (0.1 mg/L) and super-therapeutic (1 mg/L) urine levels of tiopronin and a synthetically prepared tiopronin-cysteine complex. Neat and spiked samples were assayed by the cyanide-nitroprusside assay (modified for microplate settings) and by LC-MSMS.

Results

Tiopronin increased urinary cystine by the colorimetric assay but appropriately decreased it by LC-MSMS (Table). Tiopronin-cysteine complexes also increased values in the colorimetric assay, but had no effect on the LC-MSMS assay.

Conclusion

Tiopronin caused significant interference with the colorimetric assay, even when added as a tiopronin-cysteine complex. The underlying chemistry of the method, which effectively measures reduced cysteine monomers, is the likely reason. Conversely, LC-MSMS accurately detects a fall in intact cystine when tiopronin is added to urine, and is not affected by the resulting cysteine-tiopronin complexes. Thus patients on tiopronin should ideally be monitored using an MS-based assay of intact urinary cystine to ensure proper drug dosing and disease monitoring.

Spike MaterialLevel (mg/L)AssayMean Difference (mg/L; %)95%CI (mg/L; %)
Tiopronin0.1Colorimetric2480%0 to 4826% to 135%
LC-MSMS-83-32%-154 to -12-55% to -8%
1Colorimetric323906%290 to 356654% to 1158%
LC-MSMS-218-96%-350 to -86-98% to -95%
Tiopronin + Cysteine0.1Colorimetric2862%7 to 4831% to 93%
LC-MSMS-348%-87 to 19-22% to 39%
1Colorimetric234645%181 to 286475% to 815%
LC-MSMS-1911%-61 to 22-31% to 54%