The sFlt-1/PlGF ratio immunoassay has become a cornerstone in the short-term prediction and management of preeclampsia. Clinicians frequently use this biomarker ratio to rule out the condition within one to four weeks. However, significant variability exists between different laboratory platforms. This variability can lead to clinical misinterpretation if clinicians apply universal cutoffs across non-standardized assays.

Variability in sFlt-1/PlGF Ratio Immunoassay Performance

A recent study compared the Elecsys and Kryptor immunoassays in 160 pregnant women with suspected preeclampsia. The findings revealed that the Kryptor platform consistently yielded lower concentrations for both placental growth factor (PlGF) and soluble fms-like tyrosine kinase-1 (sFlt-1). Specifically, the median PlGF was 103.5 ng/L on Kryptor versus 146.3 ng/L on Elecsys. Similarly, sFlt-1 medians were 3199 ng/L and 4034 ng/L, respectively. Consequently, using the standard Elecsys-derived cutoff of 38 on Kryptor results led to a misclassification rate of 11.9%.

The Need for Assay-Specific Cutoffs

While both assays demonstrate high diagnostic performance, they are not interchangeable. The research indicates that a Kryptor-specific threshold of 30 provides better concordance (95.0%) than the traditional Elecsys-based cutoff of 38 (88.1%). Therefore, laboratories must communicate which platform they use. Furthermore, clinicians should only apply cutoffs that have been validated for that specific hardware. In the Indian context, where maternal health outcomes are critical, ensuring the accuracy of risk stratification is paramount to prevent unnecessary hospitalizations or missed diagnoses.

Clinical Conclusion

The sFlt-1/PlGF ratio remains a robust tool for assessing angiogenic imbalance. Nevertheless, the lack of international standardization requires a cautious approach. Transitioning between platforms requires a re-evaluation of local protocols to maintain diagnostic integrity. Additionally, doctors should consider these laboratory differences when interpreting results from different diagnostic centers.

Frequently Asked Questions

Why do sFlt-1/PlGF results vary between labs?

Results vary because manufacturers use different antibodies and calibration methods. Since there is no international standard, the absolute values from one platform, such as Kryptor, are often lower than those from others like Elecsys.

Can I use the same cutoff of 38 for all preeclampsia blood tests?

No, you should not use a universal cutoff. Research shows that using the Elecsys-derived cutoff of 38 on a Kryptor assay can misclassify nearly 12% of patients. A lower cutoff, such as 30, is often more appropriate for the Kryptor platform.

Disclaimer: This content is for informational and educational purposes only and does not constitute medical advice. Always seek the advice of a qualified healthcare provider regarding any medical condition. Refer to the latest local and national guidelines for clinical practice.

References

1. Ramírez C et al. Impact of Immunoassay Selection on the sFlt-1/PlGF Ratio and Preeclampsia Risk Classification. J Appl Lab Med. 2026 May 28. doi: undefined. PMID: 42207528.


2. Zeisler H, et al. Predictive Value of the sFlt-1:PlGF Ratio in Women with Suspected Preeclampsia. N Engl J Med. 2016;374(1):13-22.


3. Yadav S, et al. Model for Early Prediction of Preeclampsia: A Nested Case Controlled Study in Indian Women. J Obstet Gynaecol India. 2020.