Icteric plasma samples, common in liver diseases, may interfere with photometric clot detection. This study was aimed to compare prothrombin time (PT) and activated partial thrompoblastin time (APTT) from non-icteric and icteric plasma samples using Werfen ACL TOP 750 and Stago sthemO 301.

Methods

We analyzed 100 routine plasma samples (50 icteric, 50 non-icteric) for PT and APTT using ACL TOP 750 (photometric clot detection) and sthemO 301 (mechanical clot detection). PT and APTT were measured in duplicate, and the icteric index (I-index) was recorded. Inter-analyzer agreement was assessed with correlation analysis, linear fit, bootstrap analysis and hypothesis testing for equality of regression slopes and intercepts.

Results

Non-icteric samples had a mean I-index of 1.0 ± 0.0 vs. 2.4 ± 0.7 in icteric samples (p<0.001). PT values showed strong inter-analyzer agreement (r=0.940 and 0.955 in non-icteric and icteric samples), while APTT exhibited moderate agreement (r=0.740 and 0.800 in non-icteric and icteric samples). Bootstrap simulations showed no significant difference in correlation coefficients between icteric and non-icteric samples. Comparison of linear fitting revealed significant differences in slopes and intercepts between non-icteric and icteric sample for both PT (slopes: p<0.001; intercepts: p=0.001) and APTT (slopes: p=0.002; intercepts: p=0.003).

Conclusions

Icteric plasma was found to modify the relationship between PT and APTT values measured by the two coagulation analyzers, indicating that values obtained with photometric and mechanical clot detection methods in icteric samples may not be directly interchangeable.

Keywords: coagulation; icterus; bilirubin; analyzers; interference

Original Paper:

Baratto, Nicola, Lorenzetto, Alessandro, Salvagno, Gian Luca and Lippi, Giuseppe. „Impact of icterus on PT and APTT measured with photometric versus mechanical clot detection“ Journal of Laboratory Medicine, vol. 50, no. 1, 2026, pp. 51-54. https://doi.org/10.1515/labmed-2025-0215