What are the limitations of the haemophilia severity?

While the haemophilia severity is highly validated, it should not replace clinical judgment. OpiCalc provides the latest evidence-based limitations for the haemophilia severity to ensure safe bedside use and optimal diagnostic accuracy.

How does the haemophilia severity compare to other Hematology scores?

The haemophilia severity is a gold-standard diagnostic tool. OpiCalc provides side-by-side clinical guidance to help clinicians choose the most specific evidence-based tool for their patient's presentation.

Is the haemophilia severity free to use?

Yes. OpiCalc provides the haemophilia severity completely ad-free and optimized for mobile bedside use, ensuring you can calculate medical risk swiftly and without distraction.

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Haemophilia Severity Classification

Activity Thresholds: WFH definitions based on baseline factor (VIII/IX) activity. While lab results define the class, the clinical phenotype determines treatment strategy.

Factor Activity (%)

Guidelines & Evidence

Clinical Details

Section 1

When to Use

Classifying the severity of newly diagnosed Haemophilia A (Factor VIII deficiency) or Haemophilia B (Factor IX deficiency).

Correlating laboratory factor activity with clinical bleeding phenotype.

Guiding the necessity for primary prophylaxis vs. episodic (on-demand) treatment.

Patient Population

Adults and children with a baseline deficiency of factor VIII or IX.

Section 2

Formula & Logic

Activity Thresholds

Severity	Factor Level (% or IU/mL)	Clinical Phenotype
Severe	< 1% (< 0.01)	Frequent spontaneous bleeds into joints or muscles; often diagnosed in infancy.
Moderate	1 - 5% (0.01 - 0.05)	Few spontaneous bleeds; prolonged bleeding with minor trauma or surgery.
Mild	6 - 40% (0.06 - 0.40)	No spontaneous bleeding; hemorrhage only with significant trauma or major surgery.

Section 3

Pearls/Pitfalls

The 'Invisible' Severe

While laboratory thresholds are standard, some patients with < 1% activity have a "moderate" clinical phenotype, while others with 2-3% activity bleed spontaneously like "severe" patients. Clinical phenotype should ultimately guide prophylaxis decisions.

Emicizumab Impact

In patients with severe Haemophilia A, Emicizumab prophylaxis can transition the bleeding clinical phenotype from "Severe" to effectively "Mild," allowing for much greater physical activity.

Section 4

Evidence Appraisal

Primary Strategy

WFH Guidelines for the Management of Hemophilia, 3rd edition.

Srivastava A et al. • Haemophilia. 2020;Global consensus on severity thresholds.

View Source

Definitions in hemophilia: on behalf of the scientific and standardization committee of the international society on thrombosis and haemostasis.

White GC et al. • Thrombosis and Haemostasis. 2001;85(3):560.

View Source

Section 5

Literature

Validation

These thresholds have been the foundation of haemophilia care since the 1950s, refined by the ISTH in 2001 to ensure uniform language in global clinical trials.

Last Comprehensive Review: 2026

Guidelines & Evidence

Clinical Details

Section 1

When to Use

Classifying the severity of newly diagnosed Haemophilia A (Factor VIII deficiency) or Haemophilia B (Factor IX deficiency).

Correlating laboratory factor activity with clinical bleeding phenotype.

Guiding the necessity for primary prophylaxis vs. episodic (on-demand) treatment.

Patient Population

Adults and children with a baseline deficiency of factor VIII or IX.

Section 2

Formula & Logic

Activity Thresholds

Severity	Factor Level (% or IU/mL)	Clinical Phenotype
Severe	< 1% (< 0.01)	Frequent spontaneous bleeds into joints or muscles; often diagnosed in infancy.
Moderate	1 - 5% (0.01 - 0.05)	Few spontaneous bleeds; prolonged bleeding with minor trauma or surgery.
Mild	6 - 40% (0.06 - 0.40)	No spontaneous bleeding; hemorrhage only with significant trauma or major surgery.

Section 3

Pearls/Pitfalls

The 'Invisible' Severe

Emicizumab Impact

In patients with severe Haemophilia A, Emicizumab prophylaxis can transition the bleeding clinical phenotype from "Severe" to effectively "Mild," allowing for much greater physical activity.

Section 4

Evidence Appraisal

Primary Strategy

WFH Guidelines for the Management of Hemophilia, 3rd edition.

Srivastava A et al. • Haemophilia. 2020;Global consensus on severity thresholds.

View Source

Definitions in hemophilia: on behalf of the scientific and standardization committee of the international society on thrombosis and haemostasis.

White GC et al. • Thrombosis and Haemostasis. 2001;85(3):560.

View Source

Section 5

Literature

Validation

These thresholds have been the foundation of haemophilia care since the 1950s, refined by the ISTH in 2001 to ensure uniform language in global clinical trials.

Last Comprehensive Review: 2026

Related Hematology Tools

Hepatic Iron Index

DAT Interpretation Algorithm

Coombs-Negative Haemolysis Checklist

PLASMIC Score

French TTP Score

PNH Clone Size Interpreter

LDH/AST Ratio

ISTH Bleeding Assessment Tool

PBT

von Willebrand Factor Activity Interpreter

Have feedback about this calculator?Let us know.

Haemophilia Severity Classification

Activity Thresholds: WFH definitions based on baseline factor (VIII/IX) activity. While lab results define the class, the clinical phenotype determines treatment strategy.

Factor Activity (%)

Guidelines & Evidence

Clinical Details

Section 1

When to Use

Classifying the severity of newly diagnosed Haemophilia A (Factor VIII deficiency) or Haemophilia B (Factor IX deficiency).

Correlating laboratory factor activity with clinical bleeding phenotype.

Guiding the necessity for primary prophylaxis vs. episodic (on-demand) treatment.

Patient Population

Adults and children with a baseline deficiency of factor VIII or IX.

Section 2

Formula & Logic

Activity Thresholds

Severity	Factor Level (% or IU/mL)	Clinical Phenotype
Severe	< 1% (< 0.01)	Frequent spontaneous bleeds into joints or muscles; often diagnosed in infancy.
Moderate	1 - 5% (0.01 - 0.05)	Few spontaneous bleeds; prolonged bleeding with minor trauma or surgery.
Mild	6 - 40% (0.06 - 0.40)	No spontaneous bleeding; hemorrhage only with significant trauma or major surgery.

Section 3

Pearls/Pitfalls

The 'Invisible' Severe

Emicizumab Impact

In patients with severe Haemophilia A, Emicizumab prophylaxis can transition the bleeding clinical phenotype from "Severe" to effectively "Mild," allowing for much greater physical activity.

Section 4

Evidence Appraisal

Primary Strategy

WFH Guidelines for the Management of Hemophilia, 3rd edition.

Srivastava A et al. • Haemophilia. 2020;Global consensus on severity thresholds.

View Source

Definitions in hemophilia: on behalf of the scientific and standardization committee of the international society on thrombosis and haemostasis.

White GC et al. • Thrombosis and Haemostasis. 2001;85(3):560.

View Source

Section 5

Literature

Validation

These thresholds have been the foundation of haemophilia care since the 1950s, refined by the ISTH in 2001 to ensure uniform language in global clinical trials.

Last Comprehensive Review: 2026

Guidelines & Evidence

Clinical Details

Section 1

When to Use

Classifying the severity of newly diagnosed Haemophilia A (Factor VIII deficiency) or Haemophilia B (Factor IX deficiency).

Correlating laboratory factor activity with clinical bleeding phenotype.

Guiding the necessity for primary prophylaxis vs. episodic (on-demand) treatment.

Patient Population

Adults and children with a baseline deficiency of factor VIII or IX.

Section 2

Formula & Logic

Activity Thresholds

Severity	Factor Level (% or IU/mL)	Clinical Phenotype
Severe	< 1% (< 0.01)	Frequent spontaneous bleeds into joints or muscles; often diagnosed in infancy.
Moderate	1 - 5% (0.01 - 0.05)	Few spontaneous bleeds; prolonged bleeding with minor trauma or surgery.
Mild	6 - 40% (0.06 - 0.40)	No spontaneous bleeding; hemorrhage only with significant trauma or major surgery.

Section 3

Pearls/Pitfalls

The 'Invisible' Severe

Emicizumab Impact

In patients with severe Haemophilia A, Emicizumab prophylaxis can transition the bleeding clinical phenotype from "Severe" to effectively "Mild," allowing for much greater physical activity.

Section 4

Evidence Appraisal

Primary Strategy

WFH Guidelines for the Management of Hemophilia, 3rd edition.

Srivastava A et al. • Haemophilia. 2020;Global consensus on severity thresholds.

View Source

Definitions in hemophilia: on behalf of the scientific and standardization committee of the international society on thrombosis and haemostasis.

White GC et al. • Thrombosis and Haemostasis. 2001;85(3):560.

View Source

Section 5

Literature

Validation

These thresholds have been the foundation of haemophilia care since the 1950s, refined by the ISTH in 2001 to ensure uniform language in global clinical trials.

Last Comprehensive Review: 2026

Related Hematology Tools

Hepatic Iron Index

DAT Interpretation Algorithm

Coombs-Negative Haemolysis Checklist

PLASMIC Score

French TTP Score

PNH Clone Size Interpreter

LDH/AST Ratio

ISTH Bleeding Assessment Tool

PBT

von Willebrand Factor Activity Interpreter

Have feedback about this calculator?Let us know.