based on the standard deviation (SD) of the response and the slope of the calibration curve. This ensures a scientifically sound and reproducible way to determine method sensitivity.

2. Scope

This procedure is applicable to the Analytical Method Development department and shall be used during method validation and verification stages, where detection limit needs to be established for drug substances, drug products, and related impurities using chromatographic techniques like HPLC, UPLC, or GC.

3. Responsibilities

• Analytical Scientist: Performs calibration experiments, calculates slope and SD, and documents LOD estimation.
• Team Lead: Verifies calculations and ensures compliance with the validation protocol.
• QA Reviewer: Reviews the data package, ensures adherence to ICH Q2(R1) standards, and approves results.

4. Accountability

The Head of Analytical Method Development is accountable for ensuring accurate implementation of this SOP and training of concerned personnel.

5. Procedure

5.1 Principles of Detection Limit Estimation

According to ICH Q2(R1), the LOD may be calculated using the formula:

LOD = (3.3 × σ) / S

• σ (sigma): Standard deviation of the response
• S: Slope of the calibration curve

5.2 Materials and Equipment

1. Calibrated HPLC/UPLC/GC system
2. Validated chromatography software (e.g., Empower, Chromeleon)
3. Working standard of the analyte
4. Mobile phase and diluent
5. Volumetric glassware and pipettes

5.3 Preparation of Calibration Solutions

1. Prepare at least 5 different concentrations near the expected LOD.
2. Inject each level in triplicate under validated chromatographic conditions.
3. Ensure linearity across the prepared concentration range.

5.4 Calculation of Standard Deviation (σ)

1. Calculate standard deviation from:
   • Blank response (if available) or
   • Y-intercepts of regression lines from multiple calibration curves
2. Alternatively, use residual standard deviation (residual SD) from calibration data points.

5.5 Calculation of Slope (S)

1. Construct the calibration curve using average peak response vs. concentration.
2. Calculate slope (S) using the least-squares method in the software or spreadsheet.

5.6 Estimation of LOD

1. Substitute σ and S in the formula: LOD = (3.3 × σ) / S
2. Record calculated LOD value and verify that signal at this concentration provides an acceptable signal-to-noise ratio (S/N ≥ 3).

5.7 Documentation Requirements

1. Record standard deviation calculation method.
2. Print calibration curve with regression equation and r² value.
3. Document LOD estimation in LOD Worksheet (Annexure-1).
4. Include method and calculations in validation report.

5.8 Deviations and Corrective Actions

1. Investigate any inconsistencies in LOD calculations (e.g., negative intercepts, poor linearity).
2. Initiate deviation report and follow CAPA process as applicable.

6. Abbreviations

• LOD: Limit of Detection
• SD: Standard Deviation
• S: Slope
• ICH: International Council for Harmonisation
• QA: Quality Assurance

7. Documents

1. Annexure-1: LOD Calculation Worksheet
2. Annexure-2: Calibration Curve Plot Template

8. References

• ICH Q2(R1) – Validation of Analytical Procedures
• USP <1058> – Analytical Instrument Qualification
• 21 CFR Part 211 – cGMP Regulations

9. SOP Version

Version: 2.0

10. Approval Section

	Prepared By	Checked By	Approved By
Signature
Date
Name	Anjali Deshmukh	Karan Joshi	Dr. Pooja Mehra
Designation	Method Development Scientist	QA Reviewer	Head – AMD
Department	Analytical Method Development	Quality Assurance	Analytical Method Development

11. Annexures

Annexure-1: LOD Calculation Worksheet

Curve ID	Standard Deviation (σ)	Slope (S)	LOD
CAL-001	0.015	0.155	0.319

Annexure-2: Calibration Curve Plot Template

Attach graphical representation of calibration curve with regression line, equation, and correlation coefficient (r² ≥ 0.99 recommended).

Revision History

Revision Date	Revision No.	Details	Reason	Approved By
01/06/2025	2.0	Clarified statistical LOD calculation and updated annexures	Annual review	Dr. Pooja Mehra
15/07/2022	1.0	Initial issue	New SOP	QA Head