Elecsys® and Kryptor immunoassays for the measurement of sFlt-1 and PlGF to aid preeclampsia diagnosis: are they comparable?

Background: For pregnant women with suspected preeclampsia, the soluble fms-like tyrosine-kinase 1 (sFlt-1)/placental growth factor (PlGF) ratio is a biomarker to aid diagnosis. We performed method comparisons between Elecsys (R) and Kryptor sFlt-1 and PlGF immunoassays and assessed the diagnostic performance for preeclampsia. Methods: Serum samples from a case-control study involving 113 pregnant women with preeclampsia/elevated liver enzymes and low platelet count (HELLP) and 270 controls were analyzed. sFlt-1 and PlGF were measured using Roche Elecsys (R) and BRAHMS Kryptor sFlt-1/PlGF immunoassays. The sFlt-1/PlGF ratios were calculated, and Passing-Bablok regression/Bland-Altman plots were performed. Gestation-specific cut-offs, <= 33 and >= 85/>= 110, were assessed. Results: Mean (+/- 2 standard deviation [SD]) differences between the Elecsys (R) and Kryptor values were: sFlt-1, 173.13 pg/mL (6237.66, -5891.40); PlGF, -102.71 pg/mL (186.06, -391.48); and sFlt-1/PlGF, 151.74 (1085.11, -781.63). The Elecsys (R) and Kryptor immunoassays showed high correlation: Pearson's correlation coefficients were 0.913 (sFlt-1) and 0.945 (PlGF). Slopes were 1.06 (sFlt-1) and 0.79 (PlGF), resulting in similar to 20% lower values for Kryptor PlGF. Sensitivities and specificities using the sFlt-1/PlGF >= 85 cut-off for early-onset preeclampsia (20 + 0 to 33 + 6 weeks) were 88.1%/100.0% (Elecsys (R)) and 90.5%/96.2% (Kryptor), respectively, and using the >= 110 cut-off for late-onset preeclampsia (>= 34 + 0 weeks) were 51.3%/96.5% (Elecsys (R)) and 78.9%/90.1% (Kryptor), respectively. Using Elecsys (R) and Kryptor sFlt-1/PlGF, 0% and 3.8% of women, respectively, were falsely ruled-in for early-onset, and 3.5% and 9.9%, respectively, for late-onset preeclampsia. Conclusions: Despite high correlation between the Elecsys (R) and Kryptor immunoassays, we observed significant differences between sFlt-1/PlGF and PlGF results. Therefore, sFlt-1/PlGF cut-offs validated for Elecsys (R) immunoassays are not transferable to Kryptor immunoassays.