Materiales de Estudio

Introduction

Order entry turns a prescriber's instructions into an accurate label, a correct quantity, and a billable claim. This is the chapter where the exam's pharmacy math lives, and math is the single most improvable area on the test because the methods repeat: once you can do days supply, ratio-and-proportion, and an IV rate, you can do most of the questions. Work every example below with a pencil. The methodology — setting up the problem and converting units before you divide — matters more than any single answer, because the exam changes the numbers but not the method.

Prescription intake and data entry

Processing begins by confirming the order is complete, legitimate, and matched to the right patient. A valid prescription shows the patient's name and address, the date issued, the drug name, strength, and quantity, complete directions for use, refill authorization, and the prescriber's name, address, and signature. Missing elements must be resolved before the prescription moves forward.

Set up the patient profile with demographics, allergies, current medications, disease states, and insurance so the system can screen for interactions and duplicate therapy. Electronic prescriptions arrive already structured, but the technician still confirms drug, strength, quantity, and directions against the transmitted record rather than assuming the data is right. Refills are dispensed only within the number and time frame authorized; anything beyond that goes to the prescriber for new authorization. On the billing side, third-party claims require the correct BIN, PCN, group number, member ID, and person code, and rejections often trace to a mismatch in one of these fields or in days supply. When a plan rejects for prior authorization, quantity limits, or non-formulary status, the technician documents the message and routes the clinical decision to the pharmacist or prescriber.

Sig codes and directions

Prescribers compress directions into abbreviations drawn largely from Latin, and the technician expands them into plain patient language stating how much, how, how often, and for what purpose when indicated.

So the sig "i tab PO BID PRN pain" translates to "Take one tablet by mouth twice daily as needed for pain," written in plain language at an accessible reading level. Note that some abbreviations are banned — reject or clarify orders using U, IU, QD, QOD, cc, or a trailing zero, because each has been repeatedly misread (ISMP List of Error-Prone Abbreviations). The sig and dosage form also drive auxiliary labels — shake well, take with food, may cause drowsiness, avoid sunlight, do not crush or chew.

Days supply and dosage calculations

Days supply drives both the label and the insurance claim, and a wrong value causes rejections or early-refill problems. The master formula is simple:

Days supply = total quantity dispensed ÷ amount used per day.

• Tablets/capsules. 60 tablets taken twice daily → 60 ÷ 2 = 30-day supply.
• Liquids. Convert the sig to mL per day, then divide the bottle volume by that number. Remember 1 teaspoonful = 5 mL and 1 tablespoonful = 15 mL.
• Insulin. Standard insulin is 100 units per mL, so a 10 mL vial contains 1000 units. A patient using 40 units/day has 1000 ÷ 40 = a 25-day supply.

Two more calculation tools cover most of the rest. Weight-based dosing: convert pounds to kilograms by dividing by 2.2, then multiply the kilogram weight by the milligrams per kilogram to get the dose. Ratio and proportion: set the known concentration equal to the unknown in the same units and cross-multiply — this handles most strength and volume conversions. Always convert units first using the metric ladder: 1 kg = 1000 g, 1 g = 1000 mg, 1 mg = 1000 mcg, 1 L = 1000 mL, and 1 fluid ounce ≈ 30 mL.

Worked example — liquid days supply. An amoxicillin suspension bottle holds 300 mL. The sig is "1 teaspoonful three times daily." One teaspoonful = 5 mL, and three times daily = 3 × 5 = 15 mL per day. Days supply = 300 ÷ 15 = 20 days.

Worked example — weight-based dose. A child weighs 44 lb; the order is 10 mg/kg per dose. Convert weight: 44 ÷ 2.2 = 20 kg. Dose = 20 kg × 10 mg/kg = 200 mg per dose.

Worked example — insulin days supply. A patient uses 25 units of insulin per day and receives one 10 mL vial. The vial holds 100 units/mL × 10 mL = 1000 units. Days supply = 1000 ÷ 25 = 40 days.

Compounding math and IV rates

Compounding requires expressing strength consistently and combining or diluting stock to reach the ordered concentration. Learn the three ways to express percent strength, because they are constantly tested:

• % weight-in-volume (w/v) = grams per 100 mL
• % weight-in-weight (w/w) = grams per 100 grams
• % volume-in-volume (v/v) = milliliters per 100 mL

A ratio strength such as 1:1000 means one gram of drug in 1000 mL of solution — which equals 1 mg per mL. (This is exactly the epinephrine-style expression the exam likes; the arithmetic is the point.)

To change a concentration by dilution, use the dilution formula: C₁ × V₁ = C₂ × V₂ (first concentration times first volume equals second concentration times second volume) to find how much stock and diluent you need. When mixing a higher and a lower strength to hit a middle strength, use alligation: subtract diagonally to find the parts of each component, then convert parts to actual quantities.

Infusions add a time dimension. The IV flow rate in mL per hour = total volume ÷ time in hours, so 1000 mL over 8 hours = 125 mL/hour. To get drops per minute, multiply the mL per minute by the administration set's drop factor (drops per mL).

Worked example — percent to grams. How many grams of dextrose are in 1000 mL of D5W (5% w/v)? 5% w/v = 5 g per 100 mL. Set up the proportion: 5 g / 100 mL = x / 1000 mL → x = 50 g.

Worked example — ratio strength. A 1:1000 solution contains how many mg per mL? 1:1000 = 1 g in 1000 mL = 1000 mg in 1000 mL = 1 mg/mL.

Worked example — IV flow rate. An order is 1 L of normal saline over 8 hours. 1 L = 1000 mL. Rate = 1000 mL ÷ 8 h = 125 mL/hour.

Worked example — dilution (C₁V₁ = C₂V₂). You have a 10% stock and need 250 mL of a 2% solution. C₁V₁ = C₂V₂ → 10% × V₁ = 2% × 250 mL → V₁ = (2 × 250) ÷ 10 = 50 mL of stock, brought up to 250 mL with 200 mL of diluent.

Business math, packaging, and labeling

Pharmacies must price, bill, and stock correctly while dispensing in compliant containers. Markup = selling price − acquisition cost, and markup percentage = that difference ÷ cost; reimbursement formulas often start from average wholesale price (AWP) minus a discount plus a dispensing fee. Gross profit = revenue − cost of goods sold, and a discount reduces the price by the stated percentage of the original amount. Inventory turnover = annual cost of goods sold ÷ average inventory value, and par levels with reorder points keep fast movers in stock without overbuying.

The finished prescription label must show the pharmacy name and address, prescription number, date, patient name, prescriber name, drug name and strength, quantity, directions, refills remaining, and the beyond-use or expiration date. Most oral prescription drugs must be dispensed in child-resistant containers unless the prescriber or patient requests otherwise in the manner the law allows (Poison Prevention Packaging Act of 1970). And a repackaged solid oral dose in a unit-dose container gets a beyond-use date no later than one year from repackaging or the manufacturer expiration date, whichever comes first, unless stability data support otherwise (USP <795>).

Worked example — markup. A drug costs the pharmacy $8.00 and sells for $12.00. Markup = $12.00 − $8.00 = $4.00. Markup percentage = $4.00 ÷ $8.00 = 0.50 = 50%.

Key numbers & facts — Order Entry

Common traps — Order Entry

• Convert units before you divide. Teaspoonfuls to mL (×5), tablespoonfuls to mL (×15), pounds to kg (÷2.2). Most wrong answers skip this.
• Insulin is 100 units/mL. A 10 mL vial is 1000 units — the exam expects this without being told.
• Percent strengths have different denominators. w/v is per 100 mL, w/w is per 100 g, v/v is per 100 mL. Mixing them up is the classic error.
• Ratio strength 1:1000 = 1 mg/mL, not 1 g/mL.
• Days supply divides total quantity by daily use — for liquids that daily use is in mL/day, not doses/day.
• Repackaging BUD caps at one year or the manufacturer date — whichever is sooner.

Self-check — Order Entry

1. A 150 mL bottle is dosed "2 teaspoonfuls twice daily." What is the days supply? (2 tsp = 10 mL; twice daily = 20 mL/day; 150 ÷ 20 = 7.5 days.)
2. Convert 165 lb to kilograms. (165 ÷ 2.2 = 75 kg.)
3. How many mg/mL is a 1:1000 solution? (1 mg/mL.)
4. Run 1500 mL over 12 hours — what is the mL/hour rate? (1500 ÷ 12 = 125 mL/hour.)
5. A 10 mL insulin vial at 100 units/mL, used 20 units/day, lasts how long? (1000 ÷ 20 = 50 days.)
6. What is the maximum beyond-use date for a repackaged unit-dose tablet? (One year from repackaging or the manufacturer expiration, whichever comes first.) (USP <795>)