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What is the Gravimetric Factor?

The gravimetric factor (GF) is a means of compensating for variances in dry dyes that may be used in the preparation of stains in the histology laboratory. The factor is calculated based on the concentration of the current dye used in a procedure and the concentration of the new dye to be placed in use in the same procedure. This allows for a better consistency in the staining process by keeping the same dye content from lot number to lot number.
The formula for determining the GF is:
Concentration of current dye used / concentration of new dye = GF
This factor is then used to multiply the amount of dye needed to prepare the staining solution. This determines the amount of new dry dye needed.
Example:
100 mL of 2.5% fast green stain is needed as a counterstain. The present dye has a dye content of 80%, and the new dye has a content of 86%. How much of the new dye (grams) is needed to prepare 100 mL of the 2.5% solution and maintain the same consistency in staining as with the old dye?
GF= 80% / 86% = 80/86 = 0.93
To prepare the stain with the old dye:
2.5% fast green dye = 2.5 g/100 mL or 2.5 g fast green dye diluted to 100 mL
To prepare the stain with the new dye:
GF X grams needed to make the staining solution = 0.93 x 2.5 g = 2.325 g
2.325 g is needed of the more concentrated new dye (86%), diluted to 100 mL to make fast green stain with the same consistency as the old fast green stain (80%). The new dye is more concentrated, so it will take less of it to have the same staining quality as the previous dye.