Modified Knott's Heartworm Technique Information

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Modified Knott's Heartworm Technique Information

Test Code: KNOTT
Test Name: Knott’s Heartworm Test

Contact Name: Dr. Mani Lejeune
Contact Telephone: 607-253-3581

Test Method: Microscopic identification
Sample Required: 1 ml of whole blood in anticoagulant, such as EDTA or heparin
Collection container: blood tube
Transport: Ship on cold packs
Test Day: M-F
Lag Time: Results same day
Species: All species that may have microfilariae in their blood
Results Format: Identification of microfilariae, or No microfilariae detected


This technique is used for the detection and identification of blood-borne microfilariae. It was developed for the detection of Dirofilaria immitis microfilariae in canine blood. In this country the microfilariae of Dirofilaria and those of Dipetalonema reconditum must be differentiated. Dirofilaria immitis microfilariae can persist in a positive animal that has been treated for up to six months after treatment.

The Knott’s Technique is not recommended as a stand alone diagnostic test for Dirofilaria immitis because infections may consist of male worms that do not produce microfilariae, or immature female worms that are not yet producing microfilariae.

For a more complete evaluation of the Dirofilaria status in a dog or cat, it is recommended that an occult serological test and a microfilarial test be done at the same time. If both techniques are done the results may be interpreted as follows:

Antigen Positive and Microfilaria Negative

  • single sex infection (female)
  • immature adult worms (5-6 months postinfection)
  • immune mediated clearance of microfilariae
  • animal on monthly preventives, or after microfilaricide treatment

Antigen Negative and Microfilaria Positive

  • microfilariae are not Dirofilaria immitis
  • heartworm antigen not present, or present in levels too low to detect
  • adult worms dead and antigen cleared, but microfilariae still present
  • microfilarial contamination of lysing solution, dye or filter chamber
  • animal transfused with microfilaremic blood
  • prenatal transfer of microfilariae
  • immune mediated clearance of antigen-antibody complexes
  • antigen destroyed due to improper storage or treatment of sample

If both a microfilarial test and an occult serological test produce inconclusive results, then other diagnostic tests may be conducted, such as thoracic radiographs, electrocardiogram, blood chemistry, and urinalysis. If the animal is clinically normal, follow up testing in six months with an occult serological test is recommended if the initial testing is inconclusive. In a positive animal that has been treated, heartworm antigen can persist for up to four months. Repeat testing must be done after that time to confirm that treatment has been effective.