Potency and Assay Testing for Compounding Pharmacies: How to Outsource with Confidence

Potency is where chemistry meets patient dose

For many compounded preparations, assay / potency testing is the clearest external check that the preparation contains the labeled amount of active pharmaceutical ingredient (API). Whether you are verifying a high-risk formula, investigating a complaint, supporting a 503B lot release strategy, or generating data for beyond-use dating decisions, the analytical partner you choose shapes both clinical confidence and inspection narratives.

This guide helps pharmacy and QA teams brief contract laboratories, avoid method mismatches, and evaluate quotes for potency work without getting lost in instrumentation jargon.

What “potency testing” usually means in practice

In outsourcing conversations, “potency” often means one of:

• A compendial assay aligned to a USP monograph (when one applies)
• A validated or verified HPLC/UPLC method for a compounded matrix
• Sometimes spectroscopic or titration methods for specific actives
• Related substances / impurities work paired with assay (especially in 503B contexts)

Be precise in your brief: “potency” alone can mean different scopes and prices.

When compounding teams send assay work out

Typical drivers:

1. New formula qualification before scaling production
2. Complaint or suspected under/over-potency
3. Supplier change for API or diluent
4. Process change (mixing time, order of addition, container)
5. Stability / BUD support studies (time-point assays)
6. Customer or quality agreement requirements (especially 503B ↔ 503A relationships)

If you only test after failures, you are using analytics as autopsy. Better programs use planned verification.

The brief that prevents method chaos

Send the lab:

• API identity and labeled strength
• Dosage form and complete formula (under CDA if needed)
• Desired acceptance range (e.g., 90.0–110.0% of labeled, or your SOP range)
• Whether you need identity confirmation in the same workflow
• Reference standard expectations (who provides USP RS / qualified standards)
• Sample quantity and light/temperature sensitivity
• Needed report elements (chromatograms on request, system suitability summary)
• Timeline and whether this is single-lot vs multi-time-point study

The more complex the matrix (creams, suspensions, transdermals, combination APIs), the more you should expect method development or verification discussion—not a same-day commodity price.

Method selection: questions that matter

Ask:

1. Do you already run this API in a similar matrix?
2. Is the method compendial, literature-based, or lab-developed?
3. What verification / validation package exists?
4. How do you handle placebo / matrix interference?
5. What are typical system suitability criteria?
6. How are OOS investigations handled for assay?

A lab that cannot explain suitability for your matrix is not ready for your high-risk formulas.

Reference standards and sample integrity

Potency disputes often start with standards and shipping:

• Confirm which reference standard grade will be used
• Confirm standard qualification if the lab uses secondary standards
• Ship with temperature control appropriate to the API
• Avoid unlabeled or poorly identified containers
• Align retain strategy before you ship the only copy of a critical lot

Chemistry accreditation signals

For assay work, look beyond a generic “ISO 17025” claim:

• Scope language covering pharmaceutical chemistry / HPLC assay as applicable
• Evidence of chromatographic competence and data integrity practices
• Clear performing site (network brands can hide transfers)

See also our guide on verifying ISO/IEC 17025 and A2LA scopes.

Potency vs stability vs dissolution—do not conflate quotes

Need	Typical ask
Strength check now	Assay / potency on current lot
How long it holds	Stability design + time-point assays
Release performance of solid oral	Dissolution / disintegration as applicable

If your true question is BUD assignment, a single potency result at t=0 is incomplete. Read our stability testing guide after this one.

How to compare potency quotes fairly

Force itemization:

• Method setup / verification fees (first-time)
• Per-sample assay fee
• Additional APIs in combination products
• Rush fees
• Impurities panel (if requested)
• Data package fees

Then score technical fit first, price second among qualified labs. Browse analytical-capable labs on LabCompare Rx and request quotes with the same formula brief.

Red flags

• Guarantees a result range before seeing the formula
• No discussion of standards
• “HPLC potency – flat price” for every cream, suspension, and injectable alike
• Unwillingness to share example system suitability language
• Vague about which site performs the chromatography

A practical onboarding sequence

1. Pick 2–3 analytically credible labs
2. Send one noncritical formula as a pilot
3. Review chromatogram clarity, turnaround honesty, and invoice surprises
4. Approve vendor in QMS
5. Graduate to higher-risk formulas

Bottom line

Potency outsourcing succeeds when pharmacies specify the clinical and quality question, and labs respond with matrix-aware methods—not generic instrument time. Treat assay partners as extensions of your quality system, especially when results will influence release, investigations, or dating.

Use LabCompare Rx to shortlist chemistry-capable CTLs and keep every bid on the same written brief.