What this case supports and what it does not

The calculations support the dilution factor and pre-dilution result, the spike recovery denominator, the point-specific bias values at 100 and 200 mg/dL, the shape of the fitted line relative to identity, and the visible +0.6 mg/dL residual outlier at 160 mg/dL. Five plotted residuals do not establish changing residual spread with concentration. They do not support an acceptance or rejection decision, because no allowable bias limit or complete comparison protocol was supplied, and the 70-99 mg/dL reference range and the 100 and 200 mg/dL decision points are not a universal laboratory range or local critical values.

Which regression procedure is appropriate, and whether an unweighted or weighted fit is justified, depends on the actual error structure of the two methods being compared; that determination belongs to a local, predefined, currently accredited protocol. The same is true for allowable bias limits, dilution and carryover acceptance criteria, and rounding and significant-figure rules, all of which are local-policy decisions that must be checked against current criteria at your laboratory.

A strong correlation tells you the two methods move together, not that they agree closely enough to substitute for one another. Calculate the bias at the concentration that matters, look at the residuals before trusting the fit, and hold the acceptance decision until a predefined local limit exists to compare it against.