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PTD - 182

Test Directory -  C1 Esterase Inhibitor



Clinical Indications

Diagnosis of hereditary angioedema (HAE). C1 esterase inhibitor deficiency is the commonest of the inherited complement deficiencies (approx. 1:50,000) and is autosomal dominant. Several members of a family are usually affected.

Test Includes

 C1 esterase inhibitor quantitative and functional activity, C3 and C4 levels.

Request Form

Combined Pathology Blood form (Yellow/Black)


Analysed by Protein Reference Unit, Sheffield if specific criteria met.

Specific Criteria

Investigation of hereditary angioedema. Usually C1 esterase inhibitor analysis will only be undertaken if C4 levels are low with normal C3 levels.
The request form should show:
- Site, duration, frequency and clinical nature of attacks
- Details of relatives similarly affected

Turnaround Time

2 weeks




2 ml


Yellow top (SST) tube or

Paediatric orange top (lithium-heparin)

Causes for Rejection

Inadequate clinical information.

Reference Range

Quantitation: 0.15 - 0.35 g/L. Functional: 40-150% (40-70% equivocal range).



C1 esterase inhibitor quantitation and functional activity should be interpreted in conjunction for diagnosis of HAE/ AAE. Reduced C1 inhibitor function may be caused by delay in sample transit and may also be caused by the use of exogeneous oestrogens, including oral contraceptive agents. C1 inhibitor is an acute-phase protein and can be increased in inflammatory processes as well as the administration of exogenous androgen and related steroids. Interpretation should consider family and clinical/ past medical history

Lab. Handling

Processing: Aliquot (MPA) and freeze at -20C in separating freezer (CB40), active frozen rack 110.
Referral:  C1 and send or NOC1 and save at -20C in separating freezer (CB40), saved frozen rack.

Version 1.0 / April 2014                                                                                                              Approved by: Consultant Biochemist