AlabamaPharmTech.com›Pharmacy Math Mastery›Lesson 6 of 6Core
Lesson 6: Sig Interpretation
Read, translate, and calculate day supply from any prescription sig — frequency abbreviations, Roman numerals, PRN dosing rules, and complex multi-phase regimens.
The sig pipeline — read it, translate it, calculate the day supply
Sig → Daily dose → Qty ÷ daily dose = day supply
Every prescription you enter follows this three-step path. Reading the sig correctly is Step 1 — everything downstream depends on it.
Frequency abbreviations
QD / QAM / QHS = once daily
BID = twice daily (every 12 hr)
TID = three times daily
QID = four times daily
Q4H = every 4 hr (6×/day)
Q6H = every 6 hr (4×/day)
Q8H = every 8 hr (3×/day)
PRN = as needed
BID = twice daily (every 12 hr)
TID = three times daily
QID = four times daily
Q4H = every 4 hr (6×/day)
Q6H = every 6 hr (4×/day)
Q8H = every 8 hr (3×/day)
PRN = as needed
Route & timing
PO = by mouth | SL = sublingual
SQ / SC = subcutaneous
IM = intramuscular
IV = intravenous
AU = both ears | AD = right ear
AS = left ear | OU = both eyes
AC = before meals | PC = after meals
gtt = drops (guttae)
SQ / SC = subcutaneous
IM = intramuscular
IV = intravenous
AU = both ears | AD = right ear
AS = left ear | OU = both eyes
AC = before meals | PC = after meals
gtt = drops (guttae)
Roman numerals in sigs
ss = ½ | i = 1 | ii = 2 | iii = 3
iv = 4 | v = 5 | vi = 6
vii = 7 | viii = 8 | x = 10
"Max viii tabs/day" = max 8 tablets/day
"i-ii tabs" = 1 to 2 tablets
Context: "iv" can be 4 OR intravenous
iv = 4 | v = 5 | vi = 6
vii = 7 | viii = 8 | x = 10
"Max viii tabs/day" = max 8 tablets/day
"i-ii tabs" = 1 to 2 tablets
Context: "iv" can be 4 OR intravenous
PRN day supply rule
Always use maximum allowed frequency for PRN medications in the day supply calculation.
CII controlled substances: use the stated max dosing (not what the patient actually takes).
"Q4-6H PRN" → use Q4H (6×/day)
"1-2 tabs" → use 2 tablets per dose
CII controlled substances: use the stated max dosing (not what the patient actually takes).
"Q4-6H PRN" → use Q4H (6×/day)
"1-2 tabs" → use 2 tablets per dose
★ PTCB tip: Sig interpretation questions appear in Order Entry & Processing. Common test formats: (1) translate a sig to plain English, (2) calculate day supply from a sig + quantity, (3) identify the maximum daily dose from a range sig, (4) flag an early refill by comparing day supply to days since fill. Know your frequency abbreviations cold — BID vs TID vs QID is tested every exam.
Most common sig errors in practice: (1) Entering BID as once daily — doubles refill frequency. (2) Not using max dosing for PRN CII — underestimates day supply, allows early fill. (3) Confusing "iv" (4 tablets) with "IV" (intravenous) — context is everything. (4) Missing the frequency divisor for complex tapers — 40 mg/day in 3 divided doses is NOT 40 mg per dose.
Six worked examples from simple sig translation through complex multi-phase day supply calculations.
EasyRetail
Rx: "ii gtt AU BID." Patient asks what this means.
Translate the sig to plain English.
Break each abbreviation:ii = 2 | gtt = drops | AU = both ears | BID = twice daily
Plain English:Instill 2 drops in both ears twice daily.
✓ Counsel patient: 2 drops, both ears, morning and night
EasyRetail
Metformin 500 mg. Sig: "1 tab PO BID." Quantity: 60 tablets.
Calculate the day supply.
Step 1 — daily dose from sig:1 tab × 2 (BID) = 2 tabs/day
Step 2 — day supply:60 ÷ 2 = 30 days
✓ Enter 30 days in the system
MediumRetail
Oxycodone 5 mg. Sig: "1-2 tabs PO Q4-6H PRN pain." Qty: 60 tabs. Schedule II.
What day supply should be entered? (CII = maximum dosing)
Max dose per administration:2 tablets
Max frequency:Q4H = every 4 hours = 6 times per day
Max daily dose:2 × 6 = 12 tablets/day
Day supply:60 ÷ 12 = 5 days
✓ Enter 5 days — CII always uses maximum dosing
MediumRetail
Albuterol HFA. Sig: "ii puffs Q4-6H PRN SOB." Canister: 200 puffs.
Day supply at maximum PRN frequency (Q4H)?
Max puffs per day:2 puffs × 6 (Q4H) = 12 puffs/day
Day supply:200 ÷ 12 = 16.67 → 16 days (round down)
✓ 16-day supply — always round DOWN for day supply
HardRetail
Medrol Dosepak: "6 tabs Day 1, 5 Day 2, 4 Day 3, 3 Day 4, 2 Day 5, 1 Day 6."
Total tablets needed and day supply?
Add each day:6 + 5 + 4 + 3 + 2 + 1 = 21 tablets
Days in course:6 days
✓ Dispense 21 tablets, enter 6-day supply — memorize this one
HardPatient scenario
TB treatment: rifampin 300 mg, "2 tabs QD × 60 days, then 1 tab QD × 120 days."
Total 300 mg tablets for the complete 6-month course?
Phase 1 (2 months = 60 days):2 tabs × 60 = 120 tablets
Phase 2 (4 months = 120 days):1 tab × 120 = 120 tablets
Total:120 + 120 = 240 tablets
✓ 240 tablets total for the complete TB course
TB treatment requires a full course — incomplete treatment causes drug-resistant TB. Dispensing the correct quantity for 6 full months is critical.
Difficulty
Context
Easy
Retail
Sig to English
Rx label reads: "1 tab PO BID."
What does this sig mean in plain English?
Answer: 1 tablet by mouth twice daily
PO = by mouth. BID = bis in die = twice per day. "1 tablet by mouth twice daily."
PO = by mouth. BID = bis in die = twice per day. "1 tablet by mouth twice daily."
Easy
Retail
Sig to English
Rx: "1-2 tabs PO Q4-6H PRN pain."
What does PRN mean?
Answer: As needed
PRN = pro re nata (Latin) = as needed. Patient takes the dose only when the symptom is present.
PRN = pro re nata (Latin) = as needed. Patient takes the dose only when the symptom is present.
Easy
Retail
Sig to English
Rx: "1 cap PO QHS."
When should the patient take this medication?
Answer: At bedtime
QHS = quaque hora somni = every hour of sleep = at bedtime. Same as QD for day supply purposes.
QHS = quaque hora somni = every hour of sleep = at bedtime. Same as QD for day supply purposes.
Med
Retail
Sig to English
Rx: "1 tab PO AC."
When does the patient take this medication?
Answer: Before meals
AC = ante cibum = before meals. PC = post cibum = after meals. Common with antacids, metformin (PC), and some diabetes medications (AC).
AC = ante cibum = before meals. PC = post cibum = after meals. Common with antacids, metformin (PC), and some diabetes medications (AC).
Med
Retail
Sig to English
Rx: "ii gtt AU BID."
What does this sig instruct the patient to do?
Answer: 2 drops in both ears twice daily
ii = 2, gtt = drops, AU = both ears (auris utraque), BID = twice daily. "Instill 2 drops in both ears twice daily." Note: AU = both, AD = right ear, AS = left ear.
ii = 2, gtt = drops, AU = both ears (auris utraque), BID = twice daily. "Instill 2 drops in both ears twice daily." Note: AU = both, AD = right ear, AS = left ear.
Hard
Retail
Sig to English
Full sig: "Take i-ii tabs PO Q4-6H PRN pain. Max iv tabs/day."
What is the maximum number of tablets the patient may take in one day?
Answer: 4 tablets
"Max iv tabs/day." iv = 4 in Roman numerals. The patient may take a maximum of 4 tablets per day regardless of frequency.
"Max iv tabs/day." iv = 4 in Roman numerals. The patient may take a maximum of 4 tablets per day regardless of frequency.
Easy
Retail
Sig → Day Supply
Sig: "1 tab PO QD." Quantity dispensed: 90 tablets.
What day supply should be entered?
Answer: 90 days
1 tab × 1 (QD) = 1 tab/day. 90 ÷ 1 = 90 days.
1 tab × 1 (QD) = 1 tab/day. 90 ÷ 1 = 90 days.
Easy
Retail
Sig → Day Supply
Sig: "1 tab PO QAM." Quantity: 30 tablets.
Day supply?
Answer: 30 days
QAM = every morning = once daily. 30 ÷ 1 = 30 days.
QAM = every morning = once daily. 30 ÷ 1 = 30 days.
Easy
Retail
Sig → Day Supply
Sig: "i tab PO BID." Quantity: 60 tablets.
Day supply?
Answer: 30 days
i = 1. 1 × 2 (BID) = 2 tabs/day. 60 ÷ 2 = 30 days.
i = 1. 1 × 2 (BID) = 2 tabs/day. 60 ÷ 2 = 30 days.
Med
Retail
Sig → Day Supply
Sig: "1 cap PO TID." Quantity: 30 capsules.
Day supply?
Answer: 10 days
1 × 3 (TID) = 3 caps/day. 30 ÷ 3 = 10 days.
1 × 3 (TID) = 3 caps/day. 30 ÷ 3 = 10 days.
Med
Retail
Sig → Day Supply
Oxycodone 5 mg. Sig: "1-2 tabs PO Q4-6H PRN pain." Qty: 60 tabs. Schedule II — use maximum dosing.
What day supply should be entered?
Answer: 5 days
Max: 2 tabs × 6 (Q4H) = 12 tabs/day. 60 ÷ 12 = 5 days. CII controlled substances always use maximum dosing for day supply.
Max: 2 tabs × 6 (Q4H) = 12 tabs/day. 60 ÷ 12 = 5 days. CII controlled substances always use maximum dosing for day supply.
Med
Retail
Sig → Day Supply
Ibuprofen 400 mg. Sig: "ii tabs PO Q6H PRN." Qty: 120 tabs.
Day supply using max frequency?
Answer: 15 days
Max: 2 × 4 (Q6H) = 8 tabs/day. 120 ÷ 8 = 15 days.
Max: 2 × 4 (Q6H) = 8 tabs/day. 120 ÷ 8 = 15 days.
Med
Retail
Sig → Day Supply
Sig: "i tab PO QD." Qty: 30 tablets.
Day supply?
Answer: 30 days
1 × 1 (QD) = 1 tab/day. 30 ÷ 1 = 30 days.
1 × 1 (QD) = 1 tab/day. 30 ÷ 1 = 30 days.
Hard
Retail
Sig → Day Supply
Sig: "1 tab PO BID × 10 days (herpes outbreak)."
How many tablets are needed for the complete course?
Answer: 20 tablets
1 × 2 (BID) × 10 days = 20 tablets.
1 × 2 (BID) × 10 days = 20 tablets.
Hard
Retail
Sig → Day Supply
Sig: "1 tab PO Q12H × 5 days, then 1 tab PO QD × 5 days."
Total tablets needed?
Answer: 15 tablets
Q12H × 5d: 2 × 5 = 10. QD × 5d: 1 × 5 = 5. Total: 10 + 5 = 15 tablets.
Q12H × 5d: 2 × 5 = 10. QD × 5d: 1 × 5 = 5. Total: 10 + 5 = 15 tablets.
Hard
Patient scenario
Sig → Day Supply
Albuterol HFA. Sig: "ii puffs Q4-6H PRN SOB." Canister: 200 puffs.
Day supply at max frequency (Q4H = 6 times/day)?
Answer: 16 days
Max puffs/day: 2 × 6 = 12. 200 ÷ 12 = 16.67 → 16 days (round down).
Max puffs/day: 2 × 6 = 12. 200 ÷ 12 = 16.67 → 16 days (round down).
Easy
Retail
Roman Numerals
Rx reads: "i tab PO QD."
How many tablets does the patient take per day?
Answer: 1 tablet
"i" in Roman numerals = 1. In pharmacy sigs, lowercase Roman numerals are used for quantities.
"i" in Roman numerals = 1. In pharmacy sigs, lowercase Roman numerals are used for quantities.
Easy
Retail
Roman Numerals
Rx reads: "take ss tab PO BID."
How many total tablets does the patient take per day?
Answer: 1 tablet
"ss" = semis = one-half = 0.5 tablet. BID = twice daily. 0.5 × 2 = 1 tablet per day.
"ss" = semis = one-half = 0.5 tablet. BID = twice daily. 0.5 × 2 = 1 tablet per day.
Easy
Retail
Roman Numerals
Sig states: give "iii gtt."
Roman numeral "iii" equals:
Answer: 3
"iii" = 3. i = 1, ii = 2, iii = 3, iv = 4, v = 5, vi = 6, vii = 7, viii = 8, ix = 9, x = 10.
"iii" = 3. i = 1, ii = 2, iii = 3, iv = 4, v = 5, vi = 6, vii = 7, viii = 8, ix = 9, x = 10.
Med
Retail
Roman Numerals
A Rx reads: "Max iv tabs/day."
What does "iv" mean in this context?
Answer: 4 — can mean tablets OR intravenous route depending on context
"iv" in Roman numerals = 4. In a tablet sig, iv = 4 tablets. But "IV" (uppercase) also means intravenous. Context determines meaning — always read the full sig carefully.
"iv" in Roman numerals = 4. In a tablet sig, iv = 4 tablets. But "IV" (uppercase) also means intravenous. Context determines meaning — always read the full sig carefully.
Med
Retail
Roman Numerals
Cough syrup Rx: "iii gtt PO Q4H PRN."
How many drops per dose?
Answer: 3 drops
"iii gtt" = 3 drops (iii = 3, gtt = drops from guttae).
"iii gtt" = 3 drops (iii = 3, gtt = drops from guttae).
Hard
Retail
Roman Numerals
Rx: "i-ii caps PO QID PRN. Max viii caps/day."
Using the stated maximum, how many capsules per day?
Answer: 8 capsules
"Max viii caps/day." viii = 8 capsules per day maximum.
"Max viii caps/day." viii = 8 capsules per day maximum.
Med
Retail
PRN Day Supply
Zolpidem 10 mg. Sig: 1 tab QHS PRN insomnia. Qty: 30 tabs. Patient reports taking it 4 nights/week.
Approximately how many weeks will 30 tablets last at 4 nights per week?
Answer: 7.5 weeks
30 tabs ÷ 4 per week = 7.5 weeks.
30 tabs ÷ 4 per week = 7.5 weeks.
Med
Retail
PRN Day Supply
Alprazolam 0.25 mg. Sig: 1 tab PO TID PRN anxiety. Qty: 30 tabs.
Day supply entered using maximum frequency?
Answer: 10 days
1 × 3 (TID) = 3 tabs/day at max. 30 ÷ 3 = 10 days.
1 × 3 (TID) = 3 tabs/day at max. 30 ÷ 3 = 10 days.
Med
Retail
PRN Day Supply
Hydrocodone/APAP 5/325 mg. Sig: 1-2 tabs Q4-6H PRN. Qty: 30 tabs. Schedule II.
CII day supply using maximum dosing (2 tabs × Q4H = 12/day)?
Answer: 2 days
Max: 2 × 6 = 12 tabs/day. 30 ÷ 12 = 2.5 → 2 days (round down).
Max: 2 × 6 = 12 tabs/day. 30 ÷ 12 = 2.5 → 2 days (round down).
Hard
Retail
PRN Day Supply
Patient fills #30 Norco (calculated day supply = 2 days at max dose) on Day 1. Returns Day 3 asking for a refill.
Can the patient refill on Day 3?
Answer: Yes — the 2-day supply elapsed by Day 3
Day supply = 2 days from Day 1. Refill eligible from Day 3 onward. Patient is on Day 3. Yes — the supply has elapsed and a new Rx may be filled. CII rules limit quantity, not necessarily the refill window on a new written Rx.
Day supply = 2 days from Day 1. Refill eligible from Day 3 onward. Patient is on Day 3. Yes — the supply has elapsed and a new Rx may be filled. CII rules limit quantity, not necessarily the refill window on a new written Rx.
Hard
Retail
PRN Day Supply
Metoprolol 25 mg #90, 1 tab QD (scheduled). Alprazolam 0.5 mg #30, 1 tab TID PRN (max dosing for day supply).
Which has the longer entered day supply, and what are the two values?
Answer: Metoprolol — 90 days vs alprazolam 10 days (max PRN)
Metoprolol: 90 ÷ 1 = 90 days. Alprazolam (max PRN): 30 ÷ 3 = 10 days. PRN medications use max frequency for day supply — 10 days is entered, even though the patient may actually make the supply last much longer.
Metoprolol: 90 ÷ 1 = 90 days. Alprazolam (max PRN): 30 ÷ 3 = 10 days. PRN medications use max frequency for day supply — 10 days is entered, even though the patient may actually make the supply last much longer.
Hard
Patient scenario
PRN Day Supply
Patient uses sliding scale (average 4 units × 4 checks/day = 16 units/day) plus 20 units basal nightly. Both from a 1,000-unit vial.
How many days does one vial last?
Answer: 27 days
Total daily: 16 (sliding scale) + 20 (basal) = 36 units/day. 1,000 ÷ 36 = 27.78 → 27 days (round down).
Total daily: 16 (sliding scale) + 20 (basal) = 36 units/day. 1,000 ÷ 36 = 27.78 → 27 days (round down).
Hard
Retail
Complex Sigs
Medrol Dosepak sig: 6 tabs Day 1, 5 tabs Day 2, 4 tabs Day 3, 3 tabs Day 4, 2 tabs Day 5, 1 tab Day 6.
Total tablets and day supply for the complete Dosepak?
Answer: 21 tablets — 6 days
6+5+4+3+2+1 = 21 tablets over 6 days. Memorize: Medrol Dosepak = 21 tablets, 6 days.
6+5+4+3+2+1 = 21 tablets over 6 days. Memorize: Medrol Dosepak = 21 tablets, 6 days.
Hard
Retail
Complex Sigs
Rx: "2 tabs BID × 7 days, then 1 tab BID × 7 days."
Total tablets needed for the complete course?
Answer: 42 tablets
Phase 1: 2 × 2 × 7 = 28. Phase 2: 1 × 2 × 7 = 14. Total: 28 + 14 = 42 tablets.
Phase 1: 2 × 2 × 7 = 28. Phase 2: 1 × 2 × 7 = 14. Total: 28 + 14 = 42 tablets.
Hard
Retail
Complex Sigs
Sig: "1-2 tabs Q4-6H PRN. Max 8 tabs in 24 hours." Qty: 60 tablets.
Day supply using the stated maximum of 8 tabs per day?
Answer: 7 days
60 ÷ 8 = 7.5 → 7 days (round down).
60 ÷ 8 = 7.5 → 7 days (round down).
Hard
Patient scenario
Complex Sigs
TB treatment: 2 tabs rifampin 300 mg QD × 2 months (60 days), then 1 tab QD × 4 months (120 days).
Total 300 mg tablets for the complete 6-month course?
Answer: 240 tablets
Phase 1: 2 × 60 = 120 tablets. Phase 2: 1 × 120 = 120 tablets. Total: 120 + 120 = 240 tablets.
Phase 1: 2 × 60 = 120 tablets. Phase 2: 1 × 120 = 120 tablets. Total: 120 + 120 = 240 tablets.
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Lesson 6 · Sig Interpretation · AlabamaPharmTech.com
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