Why days supply is the field that gets claims rejected
Days supply is how long the dispensed quantity will last at the prescribed directions. It looks trivial — divide the quantity by what is used each day — and for tablets it is. It becomes the most error-prone field on a claim as soon as the quantity and the dose are counted in different units, which is the case for four of the six common dosage forms.
Get it wrong in the low direction and the patient can refill early, which for a controlled substance is a compliance problem and for any drug is a plan audit finding. Get it wrong in the high direction and the patient runs out before the plan will pay again, which produces the call to the pharmacy at 6 p.m. on a Friday. Chain audits routinely recover money on days-supply errors, because the field is checked mechanically against the quantity and the sig.
The four hard cases all involve a unit change. Insulin is dispensed in millilitres but dosed in units, at 100 units per mL for the U-100 concentrations. Inhalers are dispensed by weight but dosed in actuations, and the labelled actuation count is the only number that matters. Eye drops are dispensed in millilitres but dosed in drops, so you need a drops-per-millilitre figure that is an estimate rather than a measurement. Topicals are dispensed in grams but dosed in fingertip units or applications, so the grams per application has to be inferred from the area treated.
One formula, four unit conversions
The core relationship never changes: days = total quantity ÷ amount used per day. What changes is what "amount" is counted in, and the discipline is to get both sides of the division into the same unit before dividing.
Tablets and capsules. Both sides are counts. One tablet twice daily is two per day, so 60 tablets is 30 days. Nothing converts.
Oral liquids. Both sides are millilitres, provided the sig is in millilitres. If the sig says teaspoonfuls, convert first: one teaspoonful is 5 mL by convention. Ten millilitres three times daily is 30 mL/day, so a 300 mL bottle is 10 days.
Insulin. Convert the package to units first. A 10 mL vial of U-100 is 10 × 100 = 1,000 units. A box of five 3 mL pens is 5 × 3 × 100 = 1,500 units. Then divide by the total daily units, adding basal and every bolus together. Do not bill by the number of pens.
Inhalers. Use the labelled actuations, typically 60, 120 or 200 depending on product. Two puffs four times daily is 8 puffs/day, so a 200-actuation inhaler is 25 days. Priming sprays are not part of the labelled count, which is one reason inhalers run out slightly early in practice.
Eye and ear drops. Convert millilitres to drops, then divide. The USP standard medicine dropper delivers 20 drops of water per millilitre, and 20 gtt/mL is the figure most payers accept for an aqueous ophthalmic solution; viscous drops and suspensions deliver larger drops, nearer 16 per millilitre. Then multiply the daily drops by the number of eyes: OU doubles the consumption and halves the days supply, and forgetting it is the single most common drops error.
Topicals. Divide grams dispensed by grams per application times applications per day. Where the sig does not state a quantity, the fingertip unit is the usual estimate — approximately 0.5 g of cream or ointment squeezed from a 5 mm nozzle along an adult index fingertip, enough to cover about two adult palm areas.
Always round down. A bottle holding 12.5 days of drops is a 12-day supply. The half day cannot be dispensed, and rounding up moves the legitimate refill date later than it should be.
Worked example: latanoprost 2.5 mL, one drop in each eye at bedtime
A prescription reads: latanoprost 0.005% ophthalmic solution, 2.5 mL, instil one drop into each eye at bedtime. The plan wants a days supply.
- Convert the bottle to drops. 2.5 mL × 20 gtt/mL = 50 drops.
- Count the daily drops. 1 drop per eye × 1 administration per day × 2 eyes = 2 drops per day.
- Divide. 50 ÷ 2 = 25 days exactly, so the billed days supply is 25.
- Consumption in dispensing units. 2 drops ÷ 20 gtt/mL = 0.1 mL per day.
- What a 30-day fill needs. 0.1 × 30 = 3.0 mL, so a single 2.5 mL bottle is short of a month and the prescriber's quantity would have to change for a 30-day cycle.
- When a refill is due. At a 75% threshold, 25 × 0.75 = 18.75 days after the fill.
Now change one thing. If the sig had read one drop into the right eye only, the daily use would be 1 drop and the days supply would double to 50 days. That single-word difference is worth 25 days of supply, and it is why the eyes-treated field exists rather than being folded into the dose.
Second case, insulin. A patient uses 26 units of glargine at bedtime and receives one 10 mL U-100 vial. The vial holds 10 × 100 = 1,000 units. 1,000 ÷ 26 = 38.46, so the days supply is 38, and about 12 units remain in the vial — not enough for a 39th day. A 30-day fill would need 26 × 30 = 780 units, which is one vial; a 90-day fill would need 2,340 units, which is three vials.
How to read the answer against plan rules
Most plans pay a maximum of 90 days in one fill, and many limit retail fills to 30 days with 90-day quantities reserved for mail order or designated retail programmes. A calculated days supply above 90 usually means the quantity should be reduced rather than the field adjusted.
Refill thresholds are percentages, not fixed days. A common maintenance-drug rule releases a refill when about 75% of the supply has been used; controlled substances typically require 85–90%, and some plans require 100% for schedule II. On a 30-day supply, 75% is day 22 or 23. Entering a days supply that is too short pulls that date forward and produces a refill-too-soon rejection on the next fill rather than this one, which is why the error is often discovered late.
The exact and billed figures differ, and the remainder is real medication. A 12.5-day bottle of drops leaves half a day's worth behind. Over a year of monthly refills that leftover accumulates, and it is one reason patients on drops often have several partly used bottles.
As-needed sigs need a stated maximum. "One to two tablets every four to six hours as needed" has no single arithmetic answer. Payers almost always require the maximum daily dose from the sig — here two tablets every four hours, or twelve a day — so 60 tablets is a five-day supply. Using the minimum instead inflates the days supply and creates the refill-too-soon problem described above.
Tapers and variable regimens must be summed, not averaged from the first line. A prednisone taper of 40 mg for three days, 20 mg for three days, then 10 mg for three days uses different quantities each period; add the tablets used in each phase and divide into the total dispensed.
Days supply for common prescriptions
| Prescription | Quantity in dose units | Used per day | Days supply |
|---|---|---|---|
| 60 tablets, 1 tablet BID | 60 tablets | 2 tablets | 30 |
| 90 capsules, 1 capsule TID | 90 capsules | 3 capsules | 30 |
| 300 mL suspension, 10 mL TID | 300 mL | 30 mL | 10 |
| 10 mL U-100 vial, 26 units daily | 1,000 units | 26 units | 38 |
| Five 3 mL U-100 pens, 40 units daily | 1,500 units | 40 units | 37 |
| 200-actuation inhaler, 2 puffs QID | 200 actuations | 8 actuations | 25 |
| 2.5 mL drops, 1 gtt OU at bedtime | 50 drops | 2 drops | 25 |
| 5 mL drops, 1 gtt OU QID | 100 drops | 8 drops | 12 |
| 15 mL drops, 1 gtt OD BID | 300 drops | 2 drops | 150 |
| 30 g cream, 1 g applied TID | 30 g | 3 g | 10 |
| 60 tablets, 1 tablet every other day | 60 tablets | 0.5 tablets | 120 |
The 15 mL single-eye row shows why plan limits bite: 150 days exceeds the 90-day maximum most plans pay, so the quantity dispensed has to come down even though the calculation is correct.
The mistakes audits find
- Billing insulin by volume instead of units. A 10 mL vial is 1,000 units at U-100. Dividing 10 by the daily units gives an absurdly short days supply and an immediate refill-too-soon on the next fill.
- Ignoring OU on eye drops. Both eyes double the daily drops and halve the days supply. This is the most common drops error by a wide margin.
- Rounding the days supply up. 12.5 days is 12, not 13. Rounding up delays the legitimate refill date.
- Using the minimum dose of a PRN sig. Payers expect the maximum daily dose written in the sig. Using the minimum overstates the days supply.
- Treating teaspoonfuls as anything but 5 mL. A household teaspoon varies from about 3 to 7 mL, but the pharmaceutical convention is exactly 5 mL, and the sig should be converted on that basis.
- Using an inhaler's fill weight instead of its labelled actuations. The two are unrelated numbers, and only the actuation count divides into the daily puffs.
- Assuming every ophthalmic bottle gives 20 drops per mL. Viscous solutions and suspensions run closer to 16, which changes a 5 mL bottle from 100 drops to 80 and the days supply with it.
- Averaging a taper. Sum the tablets used in each phase of a taper rather than assuming the first line runs throughout.
Where days supply sits in the dispensing workflow
Days supply is one of four quantities the technician derives from the same sig. The others are the quantity to dispense when the prescriber writes a duration rather than a count, the refill date, and for compounded or unit-converted products the amount of each ingredient. This calculator gives the first three; the fourth is a compounding calculation.
The forms that need a unit conversion are precisely those where other pharmacy arithmetic also appears. Insulin days supply depends on total daily units, which is the same number that drives the insulin sensitivity and carb ratio calculation. Electrolyte products dispensed in milliequivalents need the mEq to mg conversion before a quantity means anything. Compounded creams and solutions prepared from two stock strengths are sized with alligation, and paediatric liquids are checked against a weight-based dose before the days supply is even relevant.
Days supply also drives regulatory limits rather than just billing ones. Many states cap an initial opioid prescription for acute pain at a stated number of days, and prescription drug monitoring programmes are queried against days supply rather than against quantity. In those settings the field is a legal statement, not an estimate, so the arithmetic and the sig it came from should both be documented.
Key terms
- Sig
- The directions for use, from the Latin signa. Every days-supply calculation starts by reading the sig into a dose and a frequency.
- Actuation
- One metered spray from an inhaler. The labelled actuation count, not the canister weight, is what divides into the daily puffs.
- U-100
- Insulin at 100 units per millilitre, the standard concentration. A 10 mL vial holds 1,000 units and a 3 mL pen holds 300.
- gtt
- Drops, from the Latin guttae. The USP standard dropper delivers 20 drops of water per millilitre; viscous ophthalmic products deliver fewer, larger drops.
- OU, OD, OS
- Both eyes, right eye, left eye. OU doubles daily consumption compared with a single eye and halves the days supply.
- Fingertip unit
- About 0.5 g of cream or ointment from a 5 mm nozzle along an adult index fingertip, covering roughly two adult palm areas. Used to estimate grams per application when the sig does not state one.
- Refill-too-soon
- A claim rejection generated when the previous fill's days supply has not been sufficiently consumed, usually at a 75% threshold for maintenance drugs and higher for controls.