Verify that you are administering the correct
dose. If the physician orders a dosage that is not
on hand, carefully calculate dosages and have
another qualified medical worker check that your
calculations are correct. What right is this?
Answers
HEY MATE,
There are 3 primary methods for calculation of medication dosages; Dimensional Analysis, Ratio Proportion, and Formula or Desired Over Have Method. We are going to explore the Desired Over Have or Formula Method, one of these 3 methods, in more detail.
Desired Over Have or Formula Method uses a formula or equation to solve for an unknown quantity (x) much like ratio proportion.
Drug calculations require the use of conversion factors, for example, when converting from pounds to kilograms or liters to milliliters. Simplistic in design, this method allows clinicians to work with various units of measurement, converting factors to find the answer. These methods are useful in checking the accuracy of the other methods of calculation, thus acting as a double or triple check.
Preparation
When clinicians are prepared and know the key conversion factors, they will be less anxious about the calculation involved. This is vital to accuracy, regardless of which formula or method employed.
Conversion Factors
1 kg = 2.2 lb
1 gallon = 4 quart
1 tsp = 5 mL
1 inch = 2.54 cm
1 L = 1,000 mL
1 kg = 1,000 g
1 oz = 30 mL = 2 tbsp
1 g = 1,000 mg
1 mg = 1,000 mcg
1 cm = 10 mm
1 tbsp = 15 mL
1 cup = 8 fl oz
1 pint = 2 cups
12 inches = 1 foot
1 L = 1.057 qt
1 lb = 16 oz
1 tbsp = 3 tsp
60 minute = 1 hour
1 cc = 1 mL
2 pints = 1 qt
8 oz = 240 mL = 1 glass
1 tsp = 60 gtt
1 pt = 500 mL = 16 oz
1 oz = 30 mL
4 oz = 120 mL (Casey, 2018).
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Technique
There are 3 primary methods for the calculation of medication dosages, as referenced above. These include Desired Over Have Method or Formula, Dimensional Analysis and Ratio and Proportion (as cited in Boyer, 2002)[Lindow, 2004].
Desired Over Have or Formula Method
Desired over Have or Formula Method is a formula or equation to solve for an unknown quantity (x) much like ratio proportion. Drug calculations require the use of conversion factors, such as when converting from pounds to kilograms or liters to milliliters. Simplistic in design, this method allows us to work with various units of measurement, converting factors to find our answer. Useful in checking the accuracy of the other methods of calculation as above mentioned, thus acting as a double or triple check.
A basic formula, solving for x, guides us in the setting up of an equation:
D/H x Q = x, or Desired dose (amount) = ordered Dose amount/amount on Hand x Quantity.
For example, a provider requests lorazepam 4 Mg IV Push for a patient in severe alcohol withdrawal. The clinician has 2 mg/mL vials on hand. How many milliliters should he or she draw up in a syringe to deliver the desired dose?
Dose ordered (4 mg) x Quantity (1 mL)/Have (2 mg) = Amount wanted to give (2 mL)
Units of measurement must match, for example, milliliters and milliliters, or one needs to convert to like units of measurement. In the example above, the ordered dose was in milligrams, and the have dose was in milligrams, both of which cancel out leaving milliliters (answer called for milliliters), so no further conversion is required.
Dimensional Analysis Method
An order placed by a provider for lorazepam 4 mg IV PUSH for CIWA score of 25 or higher, follow CAGE Protocol for subsequent dosages based on CIWA scoring.
The clinician has 2 mg/mL vials in the automated dispensing unit.
How many milliliters are needed to arrive at an ordered dose?
The desired dose os placed over 1 remember, (x mL) = 4 mg/1 x 1 mL/2 mg x (4)(1)/2 x 4/2 x 2/1 = 2 mL, keep multiplying/dividing until the desired amount is reached, 2 mL in this example.
Notice, the fraction was set up with milligrams and milligrams strategically placed so like units could cancel each other out, making the equation easier to solve for the unit desired or milliliters. The answer makes sense, so work is done.
Zeros can be canceled out in the same way as like units. For example:
1000/500 x 10/5 = 2, the 2 zeros in 1000 and 2 zeros in 500 can be crossed out since like units in numerator and denominator, leaving 10/5, a much easier fraction to solve and the answer makes sense.
We have addressed zeros, and now let us look at 1.
If one multiplies a number by a 1, then the number is unchanged.
In contrast, if you multiply a number by zero, the number becomes zero.
Examples listed below are as follows: 18 x 0 = 0 or 20 x 1 = 20.
Ratio and Proportion Method
The Ratio and Proportion Method has been around for years and is one of the oldest methods utilized in drug calculations (as cited in Boyer, 2002)[Lindow, 2004]. Addition principals is a problem-solving technique that has no bearing on this relationship, only multiplication, and division are used to navigate through a ratio and proportion problem, not adding.
Have on hand / Quantity you have = Desired Amount / x
2 mg/1 mL = 4 mg/x
2x/2 = 4/2
x = 2 mL
In colon format, one would use H:V::D:X and multiply means DV and Extremes HX.
Hx = DV, x = DV/H, 2:1::4:x, 2x = (4)(1), x = 4/2, x = 2 mL