Glucose · method comparison · Chemistry analyzer A

B Methods

B.1 Additional study details

B.2 Statistical methods applied

1. Residuals are the test result minus the result fitted by the primary regression line.
2. The fitted-response confidence band is pointwise. OLS uses Student t(n−2) with the residual SD and leverage. Deming fits use Linnet’s jackknife t(n−2) intervals. Weighted Deming iteration uses a shared concentration scale and a convergence tolerance of 10⁻¹⁰.
3. Comparative results are x and test results are y. Replicate pairs are averaged to one mean pair per specimen.
4. Ordinary least squares fits y on x. It assumes the comparative method has negligible error and the test errors have constant variance.
5. Deming regression uses λ = Var(test error)/Var(comparative error) = 1.000, with constant error variance on both axes. λ is an assumed error-variance ratio. Weighted Deming assumes spread proportional to concentration and needs results above zero. Both use jackknife t(n−2) intervals.
6. Passing–Bablok is the shifted median of pairwise slopes, with rank-based CIs. It assumes a positive linear relationship and similar error distributions on both axes.
7. The 95% limits of agreement are mean bias ± 1.96 SD. Mean bias has a 95% Student t CI, and the limits have CIs from SE = √(3s²/n). The CI is for the mean bias. The limits of agreement are for individual differences.
8. Slope, intercept and bias at each medical decision level come from the primary fit. Bias outside the studied range is inconclusive.
9. Pearson r, from the least-squares fit, describes linear association and grows with the studied range.
10. A specimen allowance below 1.96 × the SD of a matched difference from imprecision alone (the pooled repeatability SD of the two methods, from replicate pairs or linked precision studies, times √(2/k) for k replicates per specimen) is cautioned with the share of specimens imprecision alone would put outside it. The caution changes no decision.
11. Reportable-range coverage is the width of the overlap between the observed reference-axis range and the claimed reportable range, divided by the claimed range width, times 100. It describes the tested span; accuracy and verification of the reportable range are separate questions. Replicate results are averaged by specimen before determining the observed range.

B.3 Decision rules and display precision

1. Each acceptance criterion compares the unrounded estimate with its limit. Limits include their end values unless marked exclusive. A criterion with a missing estimate, too little data or a failed fit is inconclusive and does not count as met.
2. Regression lines cover the observed comparative range.
3. Entered results, claims, targets and acceptance limits are shown as entered. Calculated concentrations, SDs and fitted parameters are shown to 1 decimal place, percentages to 1 decimal place, and slopes to 3 decimal places. r and R² are shown to 4 decimal places. A value that rounds to 1 is shown as > 0.9999. Very small or large values use exponent notation at the same precision. An SD or limit that would round to zero keeps two significant digits.