5.1 Days’ Supply, Quantity, and Dose

5.1 Days’ Supply, Quantity, and Dose

Quick Answer: Days’ supply = quantity dispensed ÷ units used per day. Units used per day = dose (units per administration) × frequency (administrations per day). Reverse the formula to check quantity: quantity = days’ supply × units used per day. Example: 90 tablets, 1 tablet BID → 2 tablets/day → 45 days.

Domain 2 of the PTCB Medication Therapy Management Certificate expects technicians to calculate adherence using days’ supply, quantity, and dose. These arithmetic skills feed Proportion of Days Covered (PDC) later in this chapter, but days’ supply itself is the first checkpoint: if the claimed days’ supply does not match the label directions, refill timing and adherence flags will be wrong.

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Core Formula and Unit Discipline

Forward formula: DS = Q ÷ (D × F)

Reverse quantity check: Q = DS × (D × F)

Keep units consistent. If dose is “2 tablets” and frequency is “once daily,” U = 2 tablets/day—not “2 doses” without converting to tablets. If the patient takes half-tablets, convert to tablet fractions (0.5 tablet × 2/day = 1 tablet/day).

Frequency shortcuts used on exams and in practice

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Worked Example 1 — Classic solid oral (exam-style)

Given: 90 tablets of lisinopril 10 mg; directions: take 1 tablet by mouth twice daily (BID).

1. Units per day: 1 tablet × 2 = 2 tablets/day
2. Days’ supply: 90 ÷ 2 = 45 days

Reverse check: If the claim says “45-day supply,” expected quantity = 45 × 2 = 90 tablets. Quantity and DS agree.

Trap: Dividing 90 by 1 (ignoring BID) yields 90 days—an inflated supply that would hide early refill gaps and distort PDC.

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Worked Example 2 — TID regimen

Given: 84 capsules; take 1 capsule three times daily with meals.

1. U = 1 × 3 = 3 capsules/day
2. DS = 84 ÷ 3 = 28 days

Reverse: For a true 30-day supply at TID one capsule, Q = 30 × 3 = 90 capsules. If only 84 were dispensed, the correct days’ supply is 28, not 30—even if the software defaulted to “30.”

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Worked Example 3 — Multi-tablet dose

Given: Metformin 500 mg, quantity 180 tablets; take 2 tablets with breakfast and 1 tablet with dinner.

1. Units per day: 2 + 1 = 3 tablets/day (do not invent “TID” if meal pattern is uneven—count actual tablets)
2. DS = 180 ÷ 3 = 60 days

Reverse: A 90-day mail-order fill at this regimen needs Q = 90 × 3 = 270 tablets. Receiving only 180 for a “90-day” claim is a documentation/adherence red flag for pharmacist review.

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Worked Example 4 — Liquid antibiotic

Given: Amoxicillin suspension 400 mg/5 mL; bottle contains 100 mL; take 5 mL twice daily for 10 days.

1. U = 5 mL × 2 = 10 mL/day
2. Full-bottle theoretical DS = 100 ÷ 10 = 10 days (matches the prescribed course)
3. Quantity check for a 10-day course: Q needed = 10 × 10 = 100 mL

If the profile shows “30-day supply” for this bottle, correct it: the calculated DS is 10 days. Wrong days’ supply on short courses pollutes adherence reports when acute meds are mistakenly left in chronic measure windows (pharmacist/clinical filter), but technicians still calculate correctly from quantity and dose.

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Worked Example 5 — Every-other-day and weekly

Methotrexate example (technician math only—clinical decisions belong to the pharmacist): 12 tablets; take 2 tablets once weekly.

1. Average daily units: 2 ÷ 7 ≈ 0.286 tablets/day
2. DS = 12 ÷ (2/7) = 12 × (7/2) = 42 days (6 weeks)

Alendronate style: 4 tablets; take 1 tablet once weekly → DS = 4 × 7 = 28 days.

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Linking Days’ Supply to Refill Timing and Gap Flags

If a patient fills a 30-day supply on Day 1 and returns on Day 45 for the next fill with no overlap and no hospitalization explanation on file, there is about a 15-day gap relative to continuous daily use. Technicians use DS to estimate when the next fill should occur:

Expected next fill ≈ fill date + days’ supply (adjusted for early-refill allowances and remaining balance).

These flags prepare Targeted Medication Review (TMR) and CMR discussion—they do not authorize the technician to change directions or stop therapy.

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Common Calculation Traps

1. Ignoring frequency: 1 tab BID treated as QD.
2. Confusing strength with dose: “20 mg daily” using 10 mg tablets means 2 tablets/day, not 1.
3. PRN regimens: “1–2 tabs every 4–6 hours PRN” has no fixed DS without actual-use history; document patient-reported average use and escalate rather than inventing a false 30-day claim.
4. Insulin and pens: Variable units and device priming make simple tablet-style DS unreliable; flag for pharmacist methodology rather than forcing a fake number.
5. Software defaults: Auto “30” or “90” that ignore actual U must be overridden with calculated DS when verifying adherence inputs.

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Technician Role Boundary

Mastering days’ supply math is the foundation for PDC and for supporting Pharmacy Quality Alliance (PQA) Star Ratings gap closure in the next sections.