CGI: Clinical Global Impression

Foundational Context

The Clinical Global Impression (CGI) scale was developed by William Guy in 1976 as part of the ECDEU Assessment Manual for Psychopharmacology (NIMH). It was designed to provide a brief, standardized global measure of illness that could be used across all psychiatric conditions.

It is now used in virtually every psychiatric clinical trial worldwide as a face-valid, clinician-rated anchor for treatment outcomes.

What the Assessment Measures

The CGI consists of two primary independent ratings:

1. CGI-S (Severity of Illness): A 1–7 rating of the patient's current condition relative to the clinician's total experience with that population.
2. CGI-I (Global Improvement): A 1–7 rating of the patient's change from baseline condition.

Ratings are based on the clinician's overall judgment, integrating symptom severity, functional impact, and clinical presentation.

Interpretation Guidelines

CGI-S (Severity):

• 1: Normal, not at all ill
• 2: Borderline mentally ill
• 3: Mildly ill
• 4: Moderately ill
• 5: Markedly ill
• 6: Severely ill
• 7: Among the most extremely ill

CGI-I (Improvement):

• 1: Very much improved
• 2: Much improved
• 3: Minimally improved
• 4: No change
• 5: Minimally worse
• 6: Much worse
• 7: Very much worse

Clinical Benchmarks: A CGI-I score of 1 or 2 is the standard definition of "responder" in clinical trials. A CGI-S of 1 or 2 generally indicates remission.

Administration Considerations

• Rater: Must be completed by a clinician; not for patient self-report.
• Time: Less than 2 minutes.
• Baseline: CGI-I requires a prior CGI-S baseline rating to be validly completed.
• Global Judgment: Scores are based on overall clinical impression, not a sum of symptoms.

Psychometric Properties

• Reliability: Excellent when used by experienced clinicians.
• Validity: Strong correlation with disorder-specific rating scales (e.g., PHQ-9, PANSS).
• Sensitivity: Highly sensitive to treatment-related change across all psychiatric categories.

Limitations

• Reliant on the clinician's subjective experience and "internal norms" for that population.
• Not a substitute for detailed, disorder-specific symptom assessment.

References

Guy, W. (1976). ECDEU Assessment Manual for Psychopharmacology. US Department of Health, Education, and Welfare.

What is the Clinical Global Impression Scale?

The Clinical Global Impression (CGI) scale was developed by William Guy in 1976 as part of the ECDEU Assessment Manual for Psychopharmacology, commissioned by the National Institute of Mental Health (NIMH). Designed for rapid clinician-rated assessment, the CGI was built to provide a standardized global measure that could be used across different clinical categories and treatment settings, making it the most practical and widely adopted outcome measure in psychiatric clinical trials.

Unlike patient self-report measures such as the PHQ-9 or GAD-7, the CGI asks the clinician to make a global judgment based on their overall clinical experience with patients presenting with similar conditions. Two subscales capture different dimensions of outcome: the CGI-S (Severity of Illness) assesses current illness severity on a 1–7 scale, and the CGI-I (Global Improvement) rates total change from baseline, also on a 1–7 scale, requiring a prior CGI-S rating for meaningful comparison.

The CGI is in the public domain and free for clinical and research use. Its brevity, simplicity, and its independence from any single condition have kept it a fixture in clinical trials for nearly five decades, from antipsychotic studies in schizophrenia to SSRI trials in depression and anxiolytic research in generalized anxiety disorder. Regulators and sponsors frequently require or strongly recommend the CGI as an anchor measure alongside disorder-specific rating scales.

Clinician-rated tool. Rate current illness severity (CGI-S) and, if a baseline is available, global improvement (CGI-I).

Severity of Illness

Considering your total clinical experience with this particular population, how mentally ill is the patient at this time?

Global Improvement

Compared to the patient's condition at baseline, how much has the patient changed?

Requires a baseline CGI-S rating for valid interpretation.

Clinician reference tool only. Cannot replace individualized clinical judgment.

CGI Score Interpretation

Guy (1976) / NIMH ECDEU Assessment Manual definitions. CGI-S and CGI-I are independent ratings.

• CGI-S — Severity of Illness
• CGI-I — Global Improvement

How the CGI Works in Practice

Two subscales, one global judgment. The CGI-S can stand alone; the CGI-I always requires a prior CGI-S baseline.

CGI-S: Severity of Illness

The clinician rates the patient's current illness severity relative to their total experience with patients who have the same clinical condition. The rating is a single global judgment on the 1–7 scale, not a sum of symptoms.

Can be completed at intake or any visit without a prior score. Scores of 1–2 indicate minimal or no illness; 3–4 mild to moderate; 5–7 mark clinically significant impairment requiring active treatment.

CGI-I: Global Improvement

The clinician rates total improvement since the baseline assessment, regardless of whether it is due to drug treatment. A score of 1 ("Very much improved") or 2 ("Much improved") is typically considered a positive clinical trial response. Scores of 4 indicate no meaningful change; 5–7 indicate deterioration.

Always requires a prior CGI-S baseline rating to be interpretable. In clinical trials, CGI-I is often co-primary or key secondary endpoint alongside disorder-specific scales.

Efficacy Index (Optional Third Component)

The original CGI also included an optional Efficacy Index, a ratio of therapeutic effect to side-effect burden, scored on a 4x4 grid. This component is rarely used today because it conflates efficacy and tolerability, which most modern trial designs treat as separate endpoints. The CGI-S and CGI-I remain the standard.

When clinicians or protocols refer to "the CGI," they almost always mean CGI-S and CGI-I only. The Efficacy Index is largely a historical artifact.

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