Glossary
days supply
Days supply is how long a dispensed quantity is meant to last: the quantity divided by the amount taken per day. It comes from the sig, not from the size of the package.
Days supply is the number of days the quantity dispensed is expected to last, and it is one division: the quantity dispensed divided by the amount the patient takes each day. It comes from the sig, not from the package size, so the first step is always to turn the directions into a per-day amount.
The arithmetic is the exam's. 1 tab PO BID, #60 is 1 × 2 = 2 tablets a day, so 60 ÷ 2 = 30 days. 1 tab PO TID, #90 is 30 days as well, because 90 ÷ 3. A 15 mL bottle of a liquid dosed 5 mL QID is 15 ÷ 20 = 0.75 days — under a day, which is why quantity and days supply have to be checked against each other before the claim goes out. A PRN order has no per-day amount until the ceiling is read off the directions ("up to 4 doses a day" is the divisor), and a taper is the per-day amounts added day by day rather than one rate. Insulin and other products measured in units are the same division with units as the quantity, since the sig gives units per day.
Days supply is what a plan's limit is measured against — 30-day retail and 90-day mail patterns are the common shapes, and the rule belongs to the plan, not to a national standard. A claim whose days supply does not match the written directions is an audit finding, and a refill dispensed before the previous supply runs out is the refill-too-soon rejection. The quantity and the days supply are therefore two fields that must agree, and the technician who computes the days supply from the sig is the one who catches the mismatch.
Taught in
Related terms
- sig code — The sig is a prescription's directions, and a sig code is the shorthand written inside it — BID for twice a day, QHS for at bedtime. The pharmacy expands each code into plain English on the label.
- PRN — PRN means "as needed", from the Latin pro re nata. By itself it sets no schedule and no ceiling, so the sig must still state the dose, the shortest interval allowed, and the maximum in 24 hours.
- beyond-use date — A beyond-use date is the last date a prepared or opened medication may be used. It is set by the pharmacy from USP standards, not by the manufacturer's expiration date on the stock bottle.
Source: PTCB — PTCE Content Outline, effective January 6, 2026 (the KA 1.2 statement "Therapeutic duplications", the KA 2.2 and 2.3 statements on controlled substances and prescription requirements, the KA 4.3 and 4.4 statements on NDC, lot, expiration and returns, and the KA 3.1/3.2 error-prevention statements, quoted verbatim); FDA label and NDC data read through openFDA and DailyMed; ISMP (the high-alert medication definition and the error-prone abbreviation list); USP <795> and <797>; the DEA Pharmacist's Manual (outline accessed 2026-10-03, verbatim; every fact below restates this course's own guides, which carry the per-source citations on their own pages (FDA, ISMP, USP and DEA each publish on its own schedule))