Definition and Use
- This test is used to detect parasites circulating in peripheral blood. It should be ordered in patients when infection caused by Plasmodium species (malaria), Babesia species (babesiosis), Trypanosoma species (sleeping sickness, Chagas disease), or certain microfilaria species or systemic infection with Leishmania species is suspected. Thin and thick blood smears are prepared from free-flowing capillary blood or EDTA-anticoagulated blood. Smears are inspected after staining with Giemsa, Wright, or Wright-Giemsa stain. For positive smears, the level of parasitemia should be determined for each specimen.
- Turnaround time:
- Preliminary examination should be performed "STAT " if malaria is suspected (turnaround time <4 hours). Final report for positive smears: <24 hours.
Special Collection and Transport Instructions
- Preparation of smears at the bedside, from free-flowing capillary blood if possible, yields the best morphology. Alternatively, EDTA-anticoagulated blood may be collected. For microfilariae, the diurnal circulation of some species must be taken into account in timing specimen collection (Loa loa: 10 am " 2 pm; Wuchereria or Brugia species: 8 pm " 4 am). Transport specimens to the laboratory and prepare smears as soon as possible. In general, collect specimens on each of 3 successive days. Collect specimens every 6 " 8 hours (until positive) for optimal detection in suspected cases. Blood should be examined in treated patients after 24, 48, and 72 hours to determine effectiveness of therapy.
Interpretation
- Expected results: Negative. Sensitive detection of parasitemia may require the examination of several specimens, as recommended above.
- Positive result: Disease caused by specific parasite identified.
Limitations
- Low level of parasitemia may require the examination of multiple specimens for detection. Examination of smears prepared from buffy coat preparations may improve the sensitivity of detection for some parasites, like microfilaria and trypanosomes. The efficient detection of microfilaria requires specimen collection during the specific hours when circulation of the parasite is expected.
- Common pitfalls: Include the collection of too few specimens for examination.
Other Considerations
- In effectively treated patients, the level of parasitemia should drop very quickly. In patients with drug-resistant parasites, the level may remain stable, or even increase.
Suggested Readings
1Garcia LS. Diagnostic Parasitology, 5th ed. Washington, DC: ASM Press; 2007. 2NCCLS document M15-A. Laboratory Diagnosis of Blood-borne Parasitic Diseases; Approved Guideline. 2000. Clinical and Laboratory Standards Institute.