1.3 Calculations, Records, and Measurement Accuracy

Calculations as procedure control

Sterile compounding math is not abstract — it determines how much drug, diluent, electrolyte, base solution, or additive enters a sterile product. Perfect aseptic technique cannot rescue a CSP made at the wrong concentration or volume. PTCB's outline explicitly lists infusion times, percent solutions, dilutions, alligations, dispensing quantities, days supply, ratios and proportions, doses, concentrations, and conversions. Write units through every step — most CSPT calculation errors are unit errors.

Core calculation patterns

Worked example (dilution): You have 50% dextrose and must make 500 mL of 10% dextrose. C1V1 = C2V2 → (50%)(V1) = (10%)(500 mL) → V1 = 100 mL of 50% dextrose, diluted to 500 mL total (add 400 mL sterile water). Worked example (rate): A 1,000 mL bag must infuse over 8 hours → 1,000 ÷ 8 = 125 mL/hr. Worked example (mcg/kg/min): A 70 kg patient needs dopamine 5 mcg/kg/min from a 1,600 mcg/mL bag. Dose = 5 × 70 = 350 mcg/min = 21,000 mcg/hr; ÷ 1,600 mcg/mL ≈ 13.1 mL/hr.

Worked example (alligation): To make 70% alcohol from 95% and 50% stock, the parts are |70−50| = 20 parts of 95% and |95−70| = 25 parts of 50%, a 20:25 ratio toward each strength.

Records: planned versus actual

A master formulation record (MFR) is the approved recipe and procedure. It identifies ingredients, amounts, equipment, calculations, compounding and quality steps, packaging, storage, labeling, and beyond-use-date (BUD) logic. It is not tied to one patient.

A compounding record (CR) documents the actual event: lot numbers, expiration dates, quantities used, the compounder and verifier identities, dates and times, equipment, deviations, in-process and final checks, and the release or quarantine decision. In a recall or investigation, the CR is the traceability trail.

Documentation mistakes to catch

• The MFR was not updated after a formula or process change.
• The CR lacks lot numbers or the actual quantities used.
• A required independent calculation check is initialed but the verifying math is not shown.
• The label BUD conflicts with the approved record or the assigned <797> category.
• A deviation occurred but was not investigated or approved.

Measuring accurately

Volumetric accuracy means choosing and reading the right syringe, graduate, or pipette for the amount measured — a measured volume should fall in the middle of the device's range, never near the bottom of an oversized syringe. Watch the meniscus, air bubbles, syringe dead space, and line-of-sight parallax.

Gravimetric accuracy verifies additions by weight, comparing expected to actual mass, often using specific gravity to convert a target volume to grams (mass = volume × specific gravity). Gravimetric checks give objective evidence for each addition and are favored for high-alert ingredients and parenteral nutrition.

Weighing powders or nonsterile components demands attention to balance calibration, tare, container weight, static, powder loss, and the minimum weighable quantity. If an ingredient cannot be weighed accurately with available equipment, change the process — do not force an unreliable measurement.

Choosing the right measuring device

Select the smallest device that holds the target volume so the reading falls in the upper-middle of its scale. For example, to measure 1.4 mL, a 3 mL syringe is far more accurate than a 10 mL syringe, where each graduation is coarser. Expel air bubbles before reading, account for the small dead-space volume in the syringe hub and needle, and read the volume at the leading edge of the plunger. For graduates, read the bottom of the meniscus at eye level. These habits prevent the systematic under- or over-fill that final inspection might otherwise miss.

Final inspection

Every CSP gets a final check before release: confirm identity, concentration, container, route, label, beyond-use date, storage condition, and appearance against the order and the master formulation record. Defect signs include discoloration, visible particles, precipitate, cracks, leaks, turbidity, gas formation, coring of the stopper, damaged closures, and unexplained volume differences. The verifier should also confirm the auxiliary labeling and that the BUD matches the assigned USP <797> category and storage.

A defective or doubtful CSP is segregated or quarantined per policy with a clear status label and an investigation record. Do not relabel, shake to redissolve, re-filter, or dispense a suspicious preparation just to keep workflow moving — those actions destroy evidence and can release an unsafe product. The safe exam answer always stops release, protects the patient, escalates to the pharmacist, and documents the deviation and disposition. Traceability earned through the compounding record is what makes a later recall or root-cause investigation possible.