ICH Q3D: Guideline for Elemental Impurities in Pharmaceuticals
ICH Q3D(R2) establishes the framework for controlling elemental impurities in pharmaceuticals, classifying 24 elements into four classes (1, 2A, 2B, 3) based on toxicity and likelihood of occurrence, setting permitted daily exposure (PDE) limits by route of administration, and requiring a risk assessment-based approach to determine whether testing is needed.
ICH Q3D is the internationally harmonized guideline for controlling elemental (inorganic) impurities in drug substances and drug products. First adopted at Step 4 in December 2014, the guideline has been revised twice: Q3D(R1) in March 2019 (updating the cadmium inhalation PDE) and Q3D(R2) in April 2022 (adding cutaneous and transcutaneous routes, updating PDEs for gold, silver, nickel, and selenium, and clarifying scope for certain product types). Q3D replaced the outdated USP <231> heavy metals test, which used a non-specific sulfide precipitation method incapable of identifying or quantifying individual elements. For a practical compliance walkthrough, see our elemental impurities guide.
The transition from USP <231> to ICH Q3D represented a fundamental shift: from a single non-specific test for "heavy metals" to element-specific, risk-based control with toxicologically derived limits. This shift increased analytical specificity and sensitivity requirements but also introduced the flexibility of risk-based assessment, potentially eliminating unnecessary testing.
In this guide, you'll learn:
- The ICH Q3D classification system for elemental impurities (Class 1, 2A, 2B, 3)
- PDE values for each element across oral, parenteral, inhalation, and cutaneous routes
- The risk assessment approach and when testing is vs. is not required
- Analytical methods for elemental impurity determination (ICP-MS, ICP-OES)
- The component-based summation approach and drug product considerations
ICH Q3D Classification System
ICH Q3D Section 4 classifies 24 elemental impurities into four classes based on toxicity profile and probability of occurrence in pharmaceutical products.
Classification Rationale
| Class | Basis for Classification | Evaluation Requirement |
|---|---|---|
| Class 1 | Human toxicants with limited or no use in pharmaceuticals | Always evaluate; testing when risk assessment indicates potential presence |
| Class 2A | Route-dependent human toxicants with relatively high probability of occurrence | Always evaluate; higher likelihood of requiring testing |
| Class 2B | Route-dependent human toxicants with lower probability of occurrence | Evaluate if knowingly added or identified through risk assessment |
| Class 3 | Relatively low toxicity by oral route; significant toxicity by parenteral and inhalation routes | Evaluate for parenteral and inhalation products; generally not for oral products unless expected levels are high |
Complete Classification Table
Class 1 Elements:
| Element | Symbol | Oral PDE (ug/day) | Parenteral PDE (ug/day) | Inhalation PDE (ug/day) | Cutaneous PDE (ug/day) |
|---|---|---|---|---|---|
| Arsenic | As | 15 | 15 | 2 | 15 |
| Cadmium | Cd | 5 | 2 | 3 | 5 |
| Lead | Pb | 5 | 5 | 5 | 5 |
| Mercury | Hg | 30 | 3 | 1 | 30 |
Class 2A Elements:
| Element | Symbol | Oral PDE (ug/day) | Parenteral PDE (ug/day) | Inhalation PDE (ug/day) | Cutaneous PDE (ug/day) |
|---|---|---|---|---|---|
| Cobalt | Co | 50 | 5 | 3 | 50 |
| Nickel | Ni | 200 | 20 | 5 | 200 |
| Vanadium | V | 100 | 10 | 1 | 100 |
Class 2B Elements:
| Element | Symbol | Oral PDE (ug/day) | Parenteral PDE (ug/day) | Inhalation PDE (ug/day) | Cutaneous PDE (ug/day) |
|---|---|---|---|---|---|
| Silver | Ag | 150 | 10 | 7 | 150 |
| Gold | Au | 300 | 300 | 1 | 300 |
| Iridium | Ir | 100 | 10 | 1 | 100 |
| Osmium | Os | 100 | 10 | 1 | 100 |
| Palladium | Pd | 100 | 10 | 1 | 100 |
| Platinum | Pt | 100 | 10 | 1 | 100 |
| Rhodium | Rh | 100 | 10 | 1 | 100 |
| Ruthenium | Ru | 100 | 10 | 1 | 100 |
| Selenium | Se | 170 | 80 | 130 | 170 |
| Thallium | Tl | 8 | 8 | 8 | 8 |
Class 3 Elements:
| Element | Symbol | Oral PDE (ug/day) | Parenteral PDE (ug/day) | Inhalation PDE (ug/day) | Cutaneous PDE (ug/day) |
|---|---|---|---|---|---|
| Lithium | Li | 550 | 250 | 25 | 550 |
| Antimony | Sb | 1200 | 90 | 20 | 1200 |
| Barium | Ba | 1400 | 700 | 300 | 1400 |
| Molybdenum | Mo | 3000 | 1500 | 10 | 3000 |
| Copper | Cu | 3000 | 300 | 30 | 3000 |
| Tin | Sn | 6000 | 600 | 60 | 6000 |
| Chromium | Cr | 11000 | 1100 | 3 | 11000 |
“Note on Q3D(R2) Changes: The R2 revision (April 2022) added cutaneous and transcutaneous PDEs for all classified elements, updated the gold PDE from 100 to 300 ug/day (oral/parenteral), updated the selenium oral PDE from 150 to 170 ug/day, and updated nickel PDEs. Always reference the current R2 version for PDE values.
Scope and Applicability
Products Covered
| Product Type | Q3D Applicability | Notes |
|---|---|---|
| New drug products (NDAs) | Fully applicable | Required since effective dates |
| New drug substances | Fully applicable | Risk assessment or testing required |
| Marketed products (supplements) | Applicable upon significant change | E.g., new excipient source, manufacturing process change |
| Biologics (BLAs) | Applicable | May require adapted approach for biologic-specific considerations |
| Generic products (ANDAs) | Fully applicable | Same requirements as innovator products |
| OTC drug products | Applicable | Per regional implementation |
Products with Special Considerations
ICH Q3D Section 1.2 identifies certain product types where the guideline may not fully apply or requires adaptation:
| Product Type | Consideration |
|---|---|
| Radiopharmaceuticals | Short half-life and low frequency of administration may justify modified approach |
| Vaccines | Process-specific impurities (aluminum adjuvants) addressed separately |
| Cell and gene therapy products | Case-by-case evaluation |
| Herbal products | May have naturally occurring elevated elemental content |
| Dialysis solutions | Different exposure considerations |
| Topical products (dermal, not cutaneous route as defined in Q3D(R2)) | Q3D(R2) now addresses cutaneous route specifically |
The Risk Assessment Approach
ICH Q3D Section 5 establishes the risk assessment methodology for determining which elements require control and what level of control is appropriate.
Risk Assessment Framework
The risk assessment follows a systematic process:
Step 1: Identify Sources
| Source Category | Examples | Assessment Approach |
|---|---|---|
| Intentionally added | Catalysts (Pd, Pt, Ru), reagents | Known quantity; require control in drug substance specification |
| Drug substance manufacturing | Equipment contact (Ni, Cr, Fe from stainless steel), solvents, water | Material of construction assessment; supplier data |
| Excipients | Mineral-derived excipients (talc, kaolin), colorants (iron oxides) | Supplier COA data; excipient specifications |
| Container closure system | Glass (Li, Ba, B), rubber (Zn, Ba), plastic (catalysts) | Extractables/leachables data per ICH Q3D guidance |
| Manufacturing equipment | Reactors, mixers, mills, coating pans | Material of construction; contact time; conditions |
| Water | Purified water, WFI | Water system qualification data |
| Utilities | Compressed air, nitrogen | Utility qualification data (typically low risk) |
Step 2: Evaluate Each Source
For each identified potential source, evaluate whether the element can be present at levels that would contribute meaningfully to the PDE:
| Evaluation Method | Application |
|---|---|
| Literature data | Published studies on elemental content of excipients, equipment materials |
| Supplier data | Certificates of analysis, specifications, safety data sheets |
| Material of construction analysis | Stainless steel grades, glass types, polymer additives |
| Calculation-based assessment | Worst-case summation of potential contributions from all sources |
| Experimental testing | Direct measurement of drug substance, excipients, or drug product |
Step 3: The 30% Control Threshold
ICH Q3D Section 5.3 introduces the 30% PDE control threshold:
| Scenario | Decision |
|---|---|
| Total predicted level from all sources < 30% of PDE | Additional controls generally not needed; document risk assessment |
| Total predicted level from all sources 30-100% of PDE | Consider testing or tighter source controls; risk assessment must justify approach |
| Total predicted level from any single source > 30% of PDE | Testing of that source or component is typically required |
| Intentionally added element | Always require specification and testing regardless of level |
Step 4: Documentation
The risk assessment must be documented and available for regulatory review. Required documentation includes:
| Documentation Element | Content |
|---|---|
| Scope | Products, routes of administration, elements evaluated |
| Source identification | All potential sources considered |
| Data used | Supplier data, analytical data, literature references |
| Calculations | Worst-case summation methodology and results |
| Conclusions | Which elements require control; which are adequately controlled by existing measures |
| Control strategy | Specifications, testing frequency, acceptance criteria |
Concentration Limits in Drug Products
ICH Q3D Section 6 provides the methodology for converting PDEs (expressed in ug/day) to concentration limits in drug products (expressed in ug/g or ppm).
Calculation
Option 1: Common (Default) Limits
For products where the maximum daily dose does not exceed 10 g/day, ICH Q3D provides default concentration limits calculated using a 10 g/day divisor:
| Element | Class | Oral (ug/g) | Parenteral (ug/g) | Inhalation (ug/g) |
|---|---|---|---|---|
| As | 1 | 1.5 | 1.5 | 0.2 |
| Cd | 1 | 0.5 | 0.2 | 0.3 |
| Pb | 1 | 0.5 | 0.5 | 0.5 |
| Hg | 1 | 3.0 | 0.3 | 0.1 |
| Co | 2A | 5.0 | 0.5 | 0.3 |
| Ni | 2A | 20 | 2.0 | 0.5 |
| V | 2A | 10 | 1.0 | 0.1 |
| Pd | 2B | 10 | 1.0 | 0.1 |
| Pt | 2B | 10 | 1.0 | 0.1 |
| Tl | 2B | 0.8 | 0.8 | 0.8 |
Option 2a: Product-Specific Limits
When the daily dose is known and differs from 10 g/day, product-specific limits provide tighter or wider limits as appropriate:
Option 2b: Daily Intake Approach
For products with variable dosing, limits can be expressed as total daily intake (ug/day), compared directly to the PDE.
“Practical Preference: Option 2a is preferred for most NDA/ANDA filings because it provides limits specific to the actual drug product. Option 1 defaults are conservative for low-dose products but may be unnecessarily restrictive.
The Component-Based Summation Approach
ICH Q3D Section 7 describes the summation approach for evaluating elemental impurity contributions from individual drug product components.
Methodology
The summation approach calculates the total elemental impurity contribution by adding contributions from each component:
For each element:
Data Sources for Summation
| Component | Data Source | Typical Approach |
|---|---|---|
| Drug substance | Batch testing data or specification | Direct measurement (ICP-MS) |
| Excipients | Supplier COA or in-house testing | Request data from suppliers; test high-risk excipients |
| Water | Water system monitoring data | Use historical monitoring data |
| Container closure | Extractables study data | Extractables/leachables results |
| Equipment | Material of construction analysis | Typically covered by drug substance testing |
When Summation Suggests Testing
| Summation Result | Action |
|---|---|
| Total < 30% of PDE for all elements | Risk assessment supports no routine testing; document assessment |
| Total 30-100% of PDE for any element | Consider routine testing of the drug product or high-contributing component |
| Total > 100% of PDE for any element | Process or source control required; must reduce to below PDE |
Analytical Methods for Elemental Impurities
ICH Q3D does not prescribe specific analytical methods but requires methods capable of quantifying elements at or below the established limits. USP <233> (Elemental Impurities — Procedures) provides the harmonized analytical methodology.
Method Comparison
| Method | Detection Principle | Detection Limits | Throughput | Cost | Best Application |
|---|---|---|---|---|---|
| ICP-MS | Mass spectrometry after plasma ionization | 0.01-1 ppb | 20-40 samples/day | High ($150-300/sample) | Parenteral/inhalation products; Class 1 elements at low levels |
| ICP-OES | Optical emission after plasma excitation | 1-100 ppb | 30-50 samples/day | Medium ($75-150/sample) | Oral products; Class 3 elements; higher PDE limits |
| AAS (GFAAS/FAAS) | Atomic absorption | 0.1-10 ppb (GFAAS); 1-1000 ppb (FAAS) | 5-10 samples/day (single element) | Low per element but high for multi-element | Single-element confirmation; limited use for full panel |
| XRF | X-ray fluorescence | 1-100 ppm | High (non-destructive) | Low per analysis | Screening; incoming material testing |
ICP-MS: The Primary Method
ICP-MS is the method of choice for most pharmaceutical elemental impurity testing because:
- Sensitivity — Detects at sub-ppb levels, meeting even the lowest PDE limits (parenteral Hg at 0.3 ug/g with 10 g/day maximum dose = 0.03 ppm)
- Multi-element capability — All 24 Q3D elements in a single analysis
- Specificity — Mass-to-charge ratio provides element-specific detection
- Dynamic range — Linear range spanning several orders of magnitude
Sample preparation for ICP-MS:
| Sample Type | Digestion Method | USP <233> Reference |
|---|---|---|
| Drug substance (organic) | Closed-vessel microwave-assisted acid digestion (HNO3 + H2O2 or HNO3 + HCl) | Procedure 1 or 2 |
| Drug product (tablets, capsules) | Closed-vessel microwave-assisted acid digestion | Procedure 1 or 2 |
| Aqueous solutions | Direct analysis after acidification | Procedure 3 |
| Oils and lipids | Acid digestion or dilution in organic solvent | Procedure 1 or adapted |
Method Validation Requirements
Per USP <233> and ICH Q2(R2):
| Validation Parameter | Acceptance Criterion |
|---|---|
| Specificity | No spectral interference; interference correction applied and validated |
| LOQ | ≤ J x target concentration (J = 0.5 per USP <233>) |
| Precision (repeatability) | RSD ≤ 20% at J level |
| Accuracy (recovery, spike) | Accuracy of spiked sample: 70-150% at J level |
| Linearity | Demonstrated across working range |
| Ruggedness | Reproducibility across preparations |
Drug Product vs. Drug Substance Testing
Where to Test
| Approach | When Appropriate |
|---|---|
| Drug substance only | Element introduced during drug substance synthesis (catalysts, reagents) |
| Drug product only | Elements from excipients, container closure, or drug product manufacturing |
| Both | When contributions from multiple sources are significant |
| Neither (risk assessment only) | When risk assessment demonstrates all elements are well below 30% of PDE |
Drug Substance Specifications
For elements intentionally added during drug substance synthesis (catalysts), ICH Q3D Section 8 requires:
| Requirement | Specification Content |
|---|---|
| Identity | Element controlled (e.g., Pd, Pt) |
| Limit | PDE-based or lower if historically achievable |
| Test method | Validated ICP-MS or ICP-OES method |
| Frequency | Every batch (for intentionally added elements) |
Drug Product Specifications
ICH Q3D does not mandate routine testing of every drug product batch if:
- Risk assessment demonstrates elemental impurities are adequately controlled
- Drug substance specification controls intentionally added elements
- Excipient supplier specifications and component summation confirm levels below 30% PDE
- Periodic verification testing confirms continued compliance
“FDA Expectation: FDA's implementation guidance states that the risk assessment should be product-specific and documented. For NDAs, the risk assessment (or testing data) should be available but does not need to be included in the CTD unless testing is performed. For products where testing is part of the control strategy, the method and specification should appear in 3.2.P.5.
Key Takeaways
References
ICH Q3D was implemented with a staggered timeline. For new products (NDAs filed after the implementation date), Q3D applies at the time of filing. For marketed products, Q3D applies when a significant change triggers re-evaluation. FDA and EMA have stated that marketed products should have a risk assessment but may not require routine testing if the assessment demonstrates adequate control.