Desired Over Have Formula Practice: Tablets and Oral Liquids

Use desired over have: (D ÷ H) × Q. D, the dose prescribed, is 50 mg. H, the dose on hand, is 25 mg. Q, the quantity that holds H, is 1 tablet. D and H are already in mg, so no conversion is needed. (50 mg ÷ 25 mg) × 1 tablet = 2 tablets. Sense check: the prescribed dose is twice the tablet strength, so 2 tablets is reasonable. Pyridoxine (vitamin B6) 25 to 50 mg a day is given with isoniazid to prevent peripheral neuropathy, so the dose is in the expected range. Takeaway: confirm that D and H share a unit, divide the dose prescribed by the dose on hand, then multiply by the quantity.

Sources: StatPearls: Dose Calculation Desired Over Have Formula Method; CDC: Treatment for Drug-Susceptible Tuberculosis Disease (pyridoxine 25 to 50 mg/day with isoniazid for persons at risk of neuropathy); NIH Office of Dietary Supplements, Dietary Supplement Label Database: vitamin B6 (pyridoxine hydrochloride) 25 mg tablet

Use desired over have: (D ÷ H) × Q. D = 40 mg. H = 10 mg. Q = 1 mL, the volume that holds 10 mg. Both doses are in mg, so no conversion is needed. (40 mg ÷ 10 mg) × 1 mL = 4 mL. The answer is written as 4 mL, never 4.0 mL, because a trailing zero can be misread as 40 mL. Sense check: each mL holds 10 mg, so 40 mg needs 4 of them. The usual first dose of oral furosemide for edema is 20 to 80 mg as a single dose, so 40 mg is a reasonable prescription. Takeaway: for a liquid, Q is the volume that contains the dose on hand; here it is 1 mL.

Sources: StatPearls: Dose Calculation Desired Over Have Formula Method; Open RN Nursing Skills (ACC adaptation), 5.5 Rounding; ISMP List of Error-Prone Abbreviations, Symbols, and Dose Designations; DailyMed: Furosemide tablets label (edema in adults: usual first dose 20 to 80 mg as a single dose); DailyMed: Furosemide Oral Solution, USP label (10 mg/mL strength)

D and H must be in the same unit before dividing. Step 1: convert the prescription. 0.125 mg × 1,000 mcg/mg = 125 mcg. Step 2: desired over have. (125 mcg ÷ 250 mcg) × 1 tablet = 0.5 tablet. The tablets are scored, so a half tablet can be given. The answer is written with a leading zero: 0.5 tablet. Dividing without converting (0.125 ÷ 250) gives 0.0005 tablet, a result so small that it signals a unit mismatch. Before giving digoxin, the nurse counts the apical pulse for a full minute. Takeaway: convert first; an answer far below half a tablet or above a few tablets usually means the units do not match.

Sources: StatPearls: Dose Calculation Desired Over Have Formula Method; Open RN Nursing Skills (ACC adaptation), 5.4 Equivalencies; DailyMed: Digoxin tablets label (maintenance dose in mcg/kg/day, scored 125 mcg and 250 mcg tablets); Open RN Nursing Pharmacology 2e, 6.7 Cardiac Glycosides (apical pulse before digoxin)

Step 1: put D and H in the same unit. 0.5 g × 1,000 mg/g = 500 mg. Step 2: identify the parts. D = 500 mg, H = 250 mg, Q = 5 mL. Step 3: (500 mg ÷ 250 mg) × 5 mL = 2 × 5 mL = 10 mL. Two common errors: leaving the dose in grams, (0.5 ÷ 250) × 5 = 0.01 mL, and forgetting to multiply by Q, which gives 2 mL. Either would be a serious underdose. Sense check: the dose is double the 250 mg in each 5 mL, so the volume is double 5 mL. Takeaway: convert grams to milligrams first, and remember that Q here is 5 mL, not 1 mL.

Sources: StatPearls: Dose Calculation Desired Over Have Formula Method; Open RN Nursing Skills (ACC adaptation), 5.4 Equivalencies; DailyMed: Cephalexin for oral suspension label, section 2.1 (adult dose of 500 mg every 12 hours; 250 mg per 5 mL)

The prescription is a daily total, so find the single dose first. Step 1: 60 mEq/day ÷ 3 doses = 20 mEq per dose. Step 2: desired over have, with D = 20 mEq, H = 40 mEq and Q = 15 mL. (20 mEq ÷ 40 mEq) × 15 mL = 7.5 mL. The 473 mL is only the size of the bottle and is not used. Using the daily total as D gives 22.5 mL, which is 3 doses at once. mEq is not converted to mg; calculate with the unit on the label. The oral solution is never given undiluted: the nurse mixes each dose with at least 4 ounces (120 mL) of cold water and gives it with meals to limit stomach irritation. Takeaway: check whether the prescription is per day or per dose, and ignore numbers, such as the bottle size, that the formula does not need.

Sources: StatPearls: Dose Calculation Desired Over Have Formula Method; DailyMed: Potassium chloride oral solution label (20% solution, 40 mEq per 15 mL, 473 mL bottle; 40 to 100 mEq/day in 2 to 5 divided doses, no more than 40 mEq per dose; dilute each dose in at least 4 ounces of cold water and give with meals)