Solution. 0.8 mL
CALCULATIONS
400,000 units × mL/500,000 units = 0.8 mL
Solution. 1.5 mL
CALCULATIONS
7,500 units × mL/5,000 units = 1.5 mL
| Penicillin G sodium | 7,500 units/mL |
| Sterile water for injection (SWFI) ad | 20 mL |
The source of penicillin G sodium is a vial containing 5,000,000 units of the dry powder. The directions on the vial are to add 18 mL of diluent to obtain a concentration of 250,000 units per milliliter. How many mL of the reconstituted solution should be withdrawn to prepare the prescription?
Solution. 0.6 mL
CALCULATIONS
7,500 units/mL × 20 mL = 150,000 units needed
j = 0.6 mL
Take 0.6 mL of the reconstituted solution and add SWFI q.s. to 20 mL.
Changing Final Concentration of Reconstituted Solution when Dry Powder does not Account for Volume
A pharmacist has on hand a vial containing 200,000 units of penicillin G potassium. Assuming that the volume occupied by the penicillin G potassium is negligible, how much diluent must he add to obtain a solution containing 50,000 units/mL?
Using a vial containing 1,000 mg of methicillin sodium and sterile water for injection (SWFI) as the diluent, explain how a pharmacist would prepare the following order
Dissolve 1,000 mg vial in 5 mL SWFI, use 2 mL of reconstituted solution (=400 mg) and q.s. to 10 mL.
By dimensional analysis:
of reconstituted solution and q.s. to 10 mL with SWFI
In practice, any multiple of 2 and 5 will provide the correct reconstitution, assuming that the ratio 2:5 is maintained and remembering that the powder needs to be completely dissolved. Another limitation for reconstitution of injections is the use of syringes and needles, which limits the volumes measured.
Solution. Reconstitute 500,000 units vial with 5 mL SWFI, take 3 mL and incorporate in sufficient ointment base to make a total of 20 g. (The ointment base would have to be capable of absorbing this volume of aqueous liquid.)
CALCULATIONS
15,000 units/g × 20 g = 300,000 units needed
300,000 units needed/500,000 units available = 3/5
Thus, 3/5 of a vial is needed. If one vial is dissolved in 5 mL of diluent, 3 mL will contain 300,000 units needed for the prescription.
One could also reconstitute the powder with 10 mL and use 6 mL to obtain 300,000 units. However, the minimum volume possible should be utilized to facilitate compounding and maintain the physical characteristics of the ointment.
Solution. Reconstitute the powder with 20 mL diluent and use 7.5 mL to prepare the prescription.
CALCULATIONS
2,500 units/mL × 30 mL = 75,000 units needed
75,000 units/200,000 units = 7.5/20
The pharmacist will reconstitute a 200,000 units vial with 20 mL 0.9% NaCl, take 7.5 mL of this dilution and q.s. to 30 mL to get 2,500 units/mL required in prescription.
Changing Final Concentration of Reconstituted Solution when Dry Powder Contributes to Final Volume
The package insert of a vial containing 2 g of Rocephin® (ceftriaxone sodium) specifies that when 7.2 mL of normal saline are added to the dry powder, the final concentration is 250 mg/mL. How many milliliters of normal saline should be used to prepare the following solution?
 | Rocephin® | 2 g |
| Normal Saline | q.s. | |
| Sig. 50 mg in 1 mL by I.M. injection |
8 mL − 7.2 mL (diluent) = 0.8 mL (vol. occupied by powder)
2,000 mg × 
= 40 mL (total volume needed for Rx)
40 mL − 8 mL = 39.2 mL normal saline (to prepare the prescription)
Solution. 103 mL
CALCULATIONS
100 mL − 78 mL = 22 mL (volume occupied by the powder)
2,500 mg × 
= 125 mL of purified water
But, dry powder occupies 22 mL of volume, then,
125 mL − 22 mL = 103 mL of purified water should be added to get 100 mg/tsp.
Solution. 38 mL of SWFI
CALCULATIONS
5,000,000 units × mL/200,000 units = 25 mL (volume of reconstituted solution)
25 mL − 23 mL = 2 mL (volume occupied by the powder)
5,000,000 units × mL/125,000 units = 40 mL of SWFI
Dry powder occupies 2 mL of volume.
40 mL − 2 mL = 38 mL of SWFI should be added to get 125,000 units/mL.
Insulin, Heparin and Other Calculations Involving Units of Activity
TABLE 10.1 Some drugs expressed in units of activity and their strength equivalents (USP 26/2003, Official Monographs)
| Drug | Units of activity |
| Amoxicillin | NLT 900 μg and NMT 1050 μg Amoxicillin per mg |
| Amphotericin B | NLT 750 μg Amphotericin B per mg |
| Antihemophilic factor | NLT 100 Antihemophilic factor units per gram of protein |
| Bacitracin | NLT 40 Bacitracin units per mg |
| Cefepime | NLT 825 μg and NMT 911 μg Cefepime per mg |
| Cefuroxime | NLT 745 μg and NMT 875 μg Cefuroxime per mg |
| Chymotrypsin | NLT 1000 USP Chymotrypsin units per mg |
| Chorionic gonadotropin | NLT 1500 USP Chorionic gonadotropin units per mg |
| Clindamycin hydrochloride | NLT 800 μg of Clindamycin per mg |
| Dactinomycin | NLT 950 μg and NMT 1030 μg Dactinomycin per mg |
| Digitalis | NLT 1 USP Digitalis unit per 100 mg dried leaf of Digitalis purpurea |
| Gentamicin sulfate | NLT 590 μg of Gentamicin per mg |
| Heparin sodium | NLT 140 USP Heparin units (not equivalent to IU) per mg |
| Hyaluronidase | NMT 0.25 μg of Tyrosine per USP Hyaluronidase unit |
| Insulin | NLT 26.5 USP Insulin units* per mg |
| Insulin human | NLT 27.5 USP Insulin Human units# per mg |
| Lincomycin hydrochloride | NLT 790 μg of Lincomycin per mg |
| Mumps skin test antigen | NLT 20 CfU (complement-fixing units) per mL |
| Nystatin | NLT 4400 USP Nystatin units per mg |
| Pancreatine | NLT 25 USP units of Amylase activity, NLT 2 USP units of lipase activity and NLT 25 USP units of protease activity per mg |
| Penicillin G potassium | NLT 1440 Penicillin G units and NMT 1680 Penicillin G units per mg |
| Rubella virus vaccine live | NLT 1000 TCID50(tissue culture infectious dose) per 0.5 mL |
| Tetracycline | NLT 975 μg of Tetracycline hydrochloride per mg |
| Typhoid vaccine | 8 units per mL |
| Vancomycin hydrochloride | NLT 925 μg of Vancomycin per mg |
| Vasopressin | NLT 300 USP Vasopressin units per mg |
* 1 USP Insulin unit = 0.0342 mg of pure Insulin derived from beef or 0.0345 mg of pure Insulin derived from pork.
# 1 USP Insulin Human unit = 0.0347 mg of pure Insulin human.
Doses of drugs designated in units are prescribed in units and measured in units. As mentioned earlier in this text, it is important to always spell out the word “units” following the number, since a poorly written “U” may be mistaken for a zero.
Heparin represents a group of mucopolysaccharides that prevent or slow the formation of blood clots. Heparin is administered by intravenous (bolus or heparin drip) or deep subcutaneous route for treatment or prophylaxis of venous thrombosis. Salt forms of heparin (heparin sodium or calcium) are measured in units and commercial preparations are standardized to contain 140 USP heparin units per milligram. Dosages of heparin solutions are adjusted based on the patient’s blood coagulation tests (e.g. PTT = partial thromboplastin time) and the specific needs of the patient. For example, low doses (5,000 units, sub-Q) are used to provide effective prophylaxis for a variety of pre-surgical situations. Medium doses are used for patients with phlebitis, pulmonary emboli and during hip replacement surgery, while high doses are indicated for patients with massive pulmonary embolism. Whereas heparin is prescribed many times in a “per-day” basis or “as needed”, a weight-based heparin protocol has been the most used way of prescribing heparin. Pediatric doses are always calculated in a weight-based protocol.
Penicillins and several other antibiotics are also among the most common drugs with potencies expressed in units.
Occasionally, some biologics (diagnostic antigens, immune sera, bacterial vaccines) have strengths expressed in units of antigen per milliliter. We will discuss this subject in more detail in Chapter 12 (Immunizations).
A parenteral nutrition (PN) formulation calls for the addition of 20 units of insulin for each 100 mL of solution. A patient is to receive 1 liter of PN per day. You have in the pharmacy, 10 mL vials of Humulin® R, U-500 (500 units/mL). How many milliliters of this solution should be added to the patient’s PN to provide the required units in a daily dose?
1,000 mL × 20 units/100 mL = 200 units for daily dose
200 units × 1 mL/500 units = 0.4 mL
0.4 mL of U-500 should be added per day.
Solution. 2.5 mL
CALCULATIONS
220 lb × 1 kg/2.2 lb × 250 units/kg = 25,000 units
25,000 units × 1 mL/10,000 units = 2.5 mL
Solution.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
