OBI Medical

v-TACTM Precision

How is v-TACTM precision measured?

The standard accuracy and precision of the v-TACTM method is calculated based on data from several clinical studies collected from emergency departments, pulmonary departments and intensive care units.

A v-TACTMmeasurement (arterial values calculated from a venous blood sample and a SpO2 measurement using the v-TACTM method) is compared to arterial blood gas (ABG-1) as the golden reference. The two measurements are measured simultaneously.  

Since arterial blood gas is not a perfect measurement, but under influence for example from pre-analytical errors, a second arterial blood gas measurement (ABG-2) was measured in a clinical study, and then compared with the reference measurement (ABG-1). See figure-1 below.


Figure-1: The principle of how the v-TACTM precision has been measured


Table-1 below show the statistical variation of the v-TAC calculated arterial blood gas vs the reference arterial blood gas measurement (the blue column), while the next column show the variation between the first and the second arterial blood gas measurements.


Comments to the precision table-1:

  1. v-TAC vs ABG-1 as reference. v-TAC includes effect from variation in SpO2 measurement and all other sources of errors, such as pre-analytical errors.
  2. ‘Real-world’ variation between two independent consecutive arterial samples (same patient) and analysis (same machine). Data source: Toftegaard et al. Evaluation of a method for converting venous values of acid-base and oxygenation to arterial values
  3. pO2 variation not normally distributed

v-TAC precision plots for pH

slope = 0.967
intercept = 0.25
r = 0.966
r2 = 0.934

v-TAC precision plots for pCO2 in [mmHg]

slope = 0.988
intercept = 0.11
r = 0.988
r^2 = 0.976

v-TAC precision plots for pCO2 in [kPa]

slope = 0.988
intercept = 0.821
r = 0.988
r2 = 0.976


v-TAC precision plot for pO2 in [mmHg] & in [kPa]

X: v-TAC™ (calculation)
Y: Difference (v-TAC - Arterial)
Solid: SD line
Dash: 95% CI line

v-TAC calculate the pO2 based on the measured SpO2 and the ODC. See figure below. Due to shape of ODC, the calculated pO2 is least sensitive to variations in SpO2 at low levels (A), whereas at high levels (B) above 95%, the pO2 is more sensitive due to ODC being flat.