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The page below is a sample from the LabCE course Mathematic Calculations and Applications for the Histology Laboratory. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Use of Ratio and Proportion

Ratio and proportion calculations are used with many laboratory procedures. To make more or less of the same item (usually solutions in histology) without changing concentration (eg, molar, normal, percent), it can usually be done using ratio and proportion.
The most common method to express this is to set up an equality between the ratios written as fractions, where A is to B as C is to D or A/B = C/D.
Example 1:
How many milliliters (mL) are there in a 2 liter (L) container of phosphate buffered formaldehyde?
1 L/1,000 mL = 2 L/X mL
Cross-multiplying (1) X mL = (2)1,000
X = 2,000 mL in a 2 L container

Example 2:
2.5 grams (g) of potassium dichromate needs to be weighed out in the preparation of Orth solution. How many milligrams (mg) of potassium dichromate would that be?
1 g = 1,000 mg
2.5 g X 1,000 mg = 2,500 mg