Calcium Calculation Formula and the Routine Blood Test Method for Serum Calcium

In GB/T6436-2002 "Determination of Calcium in Feed" (EDTA complexometric method), it is mentioned that a blank test should be carried out, but the calculation formula does not subtract the blank value. I wonder why?

Calcium calculation formula:
X=V×C×40.08×1000/Vw
V - volume of EDTA standard solution consumed in titration, milliliters;
CEDTA - concentration of the standard solution, mol/L;
Vw - volume of water sample, milliliters;
40.08 - molar mass of calcium ions, g/mol.

Determination of serum calcium (methylthymol blue colorimetric method). 1. Principle: Calcium ions in serum combine with methylthymol blue (MTB) in an alkaline solution to form a blue complex. By comparing the color with a calcium standard treated in the same way, the calcium content can be calculated. 2. Reagent composition and preparation: 1. Reagent 1: MTB reagent: 40ml x 1 bottle, store at 4°C protected from light. 2. Reagent 2: alkaline solution: 100ml x 1 bottle. 3. Reagent 3: protein clarifier: 5ml x 1 bottle; it may solidify in cold weather, so heat in a water bath until transparent before use. 4. 2.5mmol/L calcium standard solution: 1ml x 2 tubes, store at 4°C protected from light. 3. Procedure: Blank tube, Standard tube, Test tube. Reagent 1: MTB reagent (ml) 1.0, 1.0, 1.0. Reagent 2: alkaline solution (ml) 2.0, 2.0, 2.0. Deionized water 10 u l. 2.5mmol/L calcium standard solution 10u l. Serum 10u l. Mix well, let stand for 5 minutes, then at a wavelength of 610mn, 1cm light path, zero with distilled water, and measure the absorbance of each tube. 4. Calculation: 1. Calculation formula: Serum calcium content (mmol/L)=(A test - A blank) / (A standard - A blank) x 2.5