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Interpretive Handbook

Test 9232 :
Filaria, Blood

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Filaria are nematodes (roundworms) which are widespread in nature, especially in tropical areas. The adults produce microscopic microfilariae which can be transmitted to humans by biting insects (mosquitos or blackflies).


In the lymphatic filariases, the microfilariae migrate to the lymphatics where they mature to adults and cause obstruction (elephantiasis).


Lymphatic filariases caused by Wuchereria bancroftii occurs primarily in Africa and that caused by Brugia malayi occurs in South and East Asia. Treatment with diethylcarbamazine or ivermectin is effective in all but the most advanced stages.


In onchocerciasis, adults mature in subcutaneous nodules and release new microfilariae which may migrate to the eye and cause blindness (African River Blindness). Ivermectin use has dramatically improved the outlook for control and treatment of onchocerciasis in Africa.

Useful For Suggests clinical disorders or settings where the test may be helpful

Detection of the microfilariae of the lymphatic filariases (not onchocerciasis) in the peripheral blood by stained smears and/or a concentration technique

Interpretation Provides information to assist in interpretation of the test results

Positive results are provided with the genus and species of the microfilariae, if identifiable.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Onchocerciasis cannot be diagnosed using blood (a "skin-snip" examination is necessary).


Microfilariae may be seen in malaria or differential count peripheral blood smears, but concentration techniques are more efficient.


Certain of the microfilariae have a nocturnal periodicity, and the blood specimen is best drawn at night, between 10 p.m. and 2 a.m.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.


If positive, organism is identified.

Clinical References Provides recommendations for further in-depth reading of a clinical nature

Grove DI: Tissue nematodes (trichinosis, dracunculiasis, filariasis). In Principles and Practice of Infectious Diseases, Fourth edition. Edited by GL Mandell, RG Douglas Jr, JE Bennett. New York, Churchill Livingstone, 1995, pp 2531-2537