Visible particulate contamination is one of the most important quality concerns in injectable pharmaceutical manufacturing.

But visual inspection is not as simple as placing a vial in front of a light and deciding whether it passes or fails.

The ability to detect a particle can change depending on its size, shape, color and density, as well as the characteristics of the drug product and its container. USP General Chapter <1790> therefore describes visual inspection as a probabilistic process rather than an inspection method with perfect detection capability.

For pharmaceutical manufacturers, this creates an important question:

How can you demonstrate that your visual inspection process—and the people or automated systems performing it—can reliably detect the defects that matter?

This is where a scientifically designed visual inspection qualification program, supported by representative defect samples, Probability of Detection (POD) studies and controlled inspection procedures, becomes important.

This guide explains the practical relationship between USP <1790>, USP <790>, visual inspection qualification, defect kits, POD and Knapp-type studies.

What Is USP <1790>?

USP General Chapter <1790> Visual Inspection of Injections provides guidance concerning the visual inspection of injectable products for visible particles.

USP explains that the chapter primarily focuses on a manual reference inspection method, while also discussing semi-automated and automated inspection methods.

One of the most important concepts in the chapter is that visual inspection is inherently probabilistic.

A particle that is detected during one inspection may not necessarily be detected every time.

Detection can be affected by factors including:

• particle size and shape;
• particle color and density;
• product characteristics;
• dosage form;
• container design;
• container transparency; and
• whether the product is inherently difficult to inspect.

USP specifically notes that some products and packages can limit inspectability compared with clear solutions presented in transparent containers.

Therefore, qualification should not be approached simply as:

“Can the inspector see a particle?”

The more useful question is:

Under defined inspection conditions, how consistently can a trained inspector or inspection system detect representative defects in this specific product-container configuration?

USP <790> vs USP <1790>: What’s the Difference?

USP <790> and USP <1790> are closely related, but they should not be treated as interchangeable.

USP <790> — Visible Particulates in Injections

USP <790> addresses visible particulate requirements for injectable drug products.

It states that products intended for parenteral administration are visually inspected for particulate matter and describes particulate matter as extraneous, mobile, undissolved particles other than gas bubbles that are unintentionally present in solutions. Examples include materials such as fibers, glass, metal, elastomeric material and precipitates. USP

USP <1790> — Visual Inspection of Injections

USP <1790>, meanwhile, provides broader guidance concerning how visual inspection is performed and understood.

It addresses the science and practical considerations behind the inspection process, including the fact that particle detection is probabilistic.

A useful way to understand the relationship is:

USP itself groups <790> and <1790> among its visible-particle standards, while <787>, <788>, <1787> and <1788> relate to subvisible particulate matter.

Why Is Pharmaceutical Visual Inspection Probabilistic?

This is perhaps the most important concept for anyone designing a visual inspection qualification program.

Imagine a small translucent particle inside a clear vial.

An inspector might detect it during one presentation but miss it during another because its orientation, movement, background contrast or position has changed.

Now change the product from a clear solution to:

• an amber container;
• an opaque or translucent formulation;
• a suspension;
• an emulsion;
• a lyophilized product;
• or another difficult-to-inspect presentation.

Detection becomes even more challenging.

USP <1790> explicitly states that the probability observed for detecting visible particles can vary according to dosage form, particle characteristics and container design.

FDA’s draft guidance on visible particulates similarly describes visual detection as probabilistic and recommends threshold studies to understand what trained personnel with suitable visual acuity can reproducibly detect for a particular product.

This is why visual inspection qualification should be based on controlled, representative challenge samples and documented performance, rather than subjective assumptions alone.

What Is a Visual Inspection Defect Kit?

A pharmaceutical visual inspection defect kit—sometimes referred to as a challenge set, qualification set or, in some contexts, a Knapp kit—is a controlled collection of samples representing defects relevant to an inspection process.

Depending on the application, a kit may include examples representing particulate or container-related defects.

For particle-related qualification, representative materials may include categories such as:

• fibers;
• glass;
• elastomeric material;
• metallic particles;
• product-related material; or
• other particles relevant to the manufacturing process.

The exact challenge set should be designed around the product, packaging system, manufacturing process, risk assessment and intended qualification activity.

This is important because a generic defect kit cannot automatically represent every product.

A clear vial containing a transparent solution, for example, presents a different inspection challenge from an amber vial or a difficult-to-inspect formulation.

What Makes a Good Qualification Defect Set?

A qualification set should be more than a collection of containers containing visible defects.

It should be designed with traceability and repeatability in mind.

Important considerations include:

1. Representative defects

The samples should reflect defects that are meaningful to the actual manufacturing and inspection process.

2. Known sample identity

Each challenge sample should be uniquely identifiable so its inspection history and intended defect can be tracked.

3. Appropriate defect characteristics

For particulate defects, relevant characteristics may include particle:

• type;
• size;
• shape;
• color;
• density; and
• contrast against the product/background.

These characteristics matter because USP <1790> identifies several of them as factors influencing detectability.

4. Suitable good units

Qualification also requires representative units without the target defects.

An inspection challenge should evaluate both the ability to detect defective units and the ability to avoid incorrectly rejecting acceptable ones.

5. Stability and traceability

Challenge units should be appropriately controlled throughout their useful life.

Any change in the sample could alter its inspection characteristics and therefore affect the validity of subsequent qualification results.

What Is Probability of Detection (POD)?

Probability of Detection (POD) expresses how likely a particular defect is to be detected under defined inspection conditions.

Instead of treating detection as a binary property—

Visible = always detected
Not visible = never detected

—POD recognizes a detection continuum.

A highly obvious particle may be detected almost every time.

A borderline particle may be detected only during some presentations.

This concept is especially useful when determining the practical detection capability of an inspection process.

A simplified example

Suppose a challenge unit is presented repeatedly during a controlled study.

If inspectors correctly identify it on 18 of 20 presentations, the observed detection frequency would be:

18 ÷ 20 = 90%

That simple calculation illustrates the concept, although an actual qualification protocol should define the statistical approach, sample design, acceptance criteria and study controls in advance.

The objective is not merely to generate a percentage.

The objective is to understand:

Which defects can this inspection process reliably detect—and under what conditions?

What Is a Knapp Study?

The term Knapp study is widely used in pharmaceutical visual inspection in connection with evaluating detection performance and establishing inspection challenge sets.

In practical application, repeated inspection results can be used to understand the detectability of individual challenge units and distinguish obvious defects from more difficult or borderline examples.

For a pharmaceutical manufacturer, this can support activities such as:

• developing qualification sets;
• evaluating inspector performance;
• assessing detection difficulty;
• supporting manual inspection programs;
• establishing challenge sets for automated inspection;
• and comparing inspection performance under controlled conditions.

However, a “Knapp kit” should not be treated as a universal off-the-shelf set that automatically qualifies every product.

The challenge material should reflect the actual inspection problem.

Inspector Qualification: What Should Be Evaluated?

A strong qualification program assesses more than whether someone can pass a single test.

It should consider whether personnel can perform inspection consistently under the defined procedure.

Relevant elements can include:

Visual capability

Personnel should be capable of performing the visual task required by the procedure.

Training

Inspectors should understand:

• the inspection technique;
• defect categories;
• product characteristics;
• inspection timing;
• handling requirements;
• acceptance/rejection procedures; and
• documentation expectations.

Representative challenge

Qualification samples should represent the kinds of defects inspectors are expected to identify.

Defined inspection conditions

Qualification should reproduce the relevant controlled conditions as closely as practical.

Documented results

Performance needs to be recorded so qualification and subsequent requalification can be demonstrated.

Manual vs Automated Visual Inspection Qualification

Visual inspection can generally involve:

Manual Visual Inspection (MVI)
A trained human inspector examines each unit.

Semi-Automated Visual Inspection (SAVI)
Equipment assists with handling or presentation while human operators make inspection decisions.

Automated Visual Inspection (AVI)
Inspection equipment uses technologies such as cameras, lighting and image-processing algorithms to evaluate units.

USP <1790> discusses manual, semi-automated and automated approaches, although its primary focus is the manual reference inspection method.

The qualification strategy consequently needs to match the inspection method.

For AVI, representative defect sets can be particularly valuable for challenging and evaluating equipment performance.

But the question remains essentially the same:

Can the inspection process reliably detect defects that are relevant to the product and manufacturing process?

Difficult-to-Inspect Products Need Special Consideration

Not every injectable product provides the same inspection conditions.

USP <1790> specifically recognizes difficult-to-inspect products (DIP) and notes that certain products or packages can limit the ability to inspect for particles.

Examples may involve products where characteristics such as opacity, color, container design or formulation behavior interfere with detection.

For these products, simply copying a qualification approach developed for a clear solution in a transparent vial may not adequately represent the inspection challenge.

The qualification strategy should therefore consider the actual product-container combination.

Visual Inspection Should Not End at Detection

Finding a visible particle is only the beginning.

FDA’s draft guidance describes a broader risk-based visible-particulate control strategy involving:

product development → manufacturing controls → visual inspection → particle identification → investigation → corrective action.

This distinction matters.

A defect kit helps evaluate whether the inspection process can detect a challenge.

Particle characterization helps answer a different question:

What is the particle, and where might it have come from?

Depending on the investigation, analytical techniques such as microscopy, FTIR or SEM-EDS may help characterize unknown particulate matter.

That creates a broader quality workflow:

Detect → Classify → Identify → Investigate → Determine Potential Source → CAPA

This is also where maintaining a reference library of known materials from product-contact components, equipment, packaging and the manufacturing environment can become valuable during root-cause investigations.

Common Visual Inspection Qualification Mistakes

1. Using an irrelevant generic defect kit

A qualification set should represent the product and inspection process being evaluated.

2. Treating visual inspection as 100% deterministic

USP <1790> explicitly characterizes visual inspection as probabilistic.

3. Focusing only on particle size

Size matters, but so do shape, color, density, product characteristics and container design.

4. Ignoring difficult-to-inspect products

Qualification conditions appropriate for clear solutions cannot automatically be generalized to every formulation.

5. Treating qualification as a one-time activity

Processes, personnel, equipment and products can change. The qualification program should define how continued competency and changes are handled.

6. Detecting particles without investigating them

FDA emphasizes identification, investigation and corrective action as components of a holistic visible-particle control strategy.

Recent FDA enforcement also illustrates the practical importance of this area. An April 2026 warning letter cited deficiencies involving personnel POD, defect categorization, particle characterization and visual-inspection procedures.

A Practical Visual Inspection Qualification Workflow

A useful framework is:

Step 1 — Define the product and container
Understand formulation, container type and inspection challenges.

Step 2 — Perform a defect risk assessment
Identify particle and non-particle defects relevant to the process.

Step 3 — Establish representative defect samples
Develop controlled challenge units and suitable good units.

Step 4 — Characterize the challenge set
Document the characteristics necessary to make the set traceable and suitable for its intended purpose.

Step 5 — Establish controlled inspection conditions
Define handling, background, illumination, inspection sequence and other relevant procedural conditions.

Step 6 — Conduct detection studies
Generate repeated observations sufficient for the qualification protocol and intended statistical assessment.

Step 7 — Establish the qualification set
Select samples appropriate for the intended qualification exercise.

Step 8 — Train and qualify inspectors or challenge the inspection system
Use the approved protocol.

Step 9 — Document and trend performance
Maintain qualification records and investigate meaningful changes.

Step 10 — Maintain the program
Define requalification, kit/sample lifecycle controls and change management.

Where Confianca Pharmazon Fits

Confianca Pharmazon supports pharmaceutical visual inspection programs through Visual Inspection Knapp/Defect Kits and associated visual inspection solutions.

Confianca’s portfolio covers visual inspection kits for multiple injectable packaging formats and also includes visual-inspection and pharmaceutical quality-support capabilities. The objective is not simply to provide “particles in vials,” but to support a controlled qualification approach aligned with the customer’s actual application.

For organizations developing or strengthening a visual inspection program, the starting information should include:

Product type → container format → inspection method → relevant defect types → intended qualification use.

From there, an appropriate defect-kit configuration can be discussed.

Need a Visual Inspection Defect Kit for Your Product?

If you are developing, validating or strengthening a manual or automated pharmaceutical visual inspection program, Confianca Pharmazon can discuss a customized visual inspection defect-kit requirement based on your product and container configuration.

→ Request a Visual Inspection Kit / Technical Discussion

Frequently Asked Questions

What is USP <1790>?

USP <1790> is a general chapter providing guidance on visual inspection of injections for visible particles. It discusses manual reference inspection as well as semi-automated and automated approaches and recognizes that visual detection is probabilistic.

What is the difference between USP <790> and USP <1790>?

USP <790> addresses visible particulate requirements for injectable products, whereas USP <1790> provides guidance concerning the visual inspection process and its application.

What is a Knapp kit?

In pharmaceutical visual inspection, the term commonly refers to a controlled set of challenge samples used in connection with visual-inspection qualification and detection-performance studies. The set should be appropriate for the specific product, container and intended inspection application.

What is POD in visual inspection?

POD means Probability of Detection. It describes the likelihood that a particular defect will be detected under specified inspection conditions.

Can the same defect kit be used for every injectable product?

Not necessarily. Detectability varies with the product, defect characteristics and container design, so qualification samples should be evaluated for their intended application.

Does USP <1790> apply only to manual inspection?

No. USP describes manual reference inspection as the primary focus of <1790>, but the chapter also discusses semi-automated and automated methods.