Arterio‐venous blood gas Δvalues for validation of umbilical cord blood samples at birth are not only biased by sample mix ups but also affected by clinical factors

Traditional validation of umbilical cord blood samples with positive veno‐arterial ΔpH and arterio‐venous Δp CO 2 values confirms the source of samples, whereas negative Δvalues represent mix‐up of samples. To investigate whether this is true, the distributions of V‐A ΔpO 2 and A‐V Δlactate were also explored and related to clinical characteristics. In addition, different cord blood sampling techniques were evaluated. Material and methods Register study with cord blood acid‐base and clinical data from 27 233 newborns. Clinical characteristics were related to positive, zero and negative Δvalues. Blood samplings from unclamped and double‐clamped cords were compared. A two‐sided P  < 0.05 was considered significant. Results ΔpH and Δp CO 2 values distributed into positive, around zero, and negative sub‐populations, with significant differences in pH and clinical characteristics between sub‐populations. No such sub‐populations were distinguished for ΔpO 2 and Δlactate. The 2.5th and 5th ΔpH percentiles were 0.013 and 0.022, respectively, and for Δp CO 2 0.30 and 0.53  kP a. Applying 5th percentile criteria resulted in 3.5% of “approved” cases showing a ΔpO 2  ≤ 0. Puncture and sampling of the unclamped cord resulted in significantly better sample quality. Conclusions Unphysiological negative ΔpO 2 values occurred despite correct validation with traditional criteria. Δlactate cannot be used for validation because both positive and negative values are physiological. Positive/around zero/negative ΔpH and Δp CO 2 sub‐populations were associated with significant differences in pH and clinical characteristics, indicating that defective sampling and sample handling are not the sole explanations for negative Δvalues. Prompt puncture and sampling of the unclamped cord resulted in best sample quality.