Thick and thin blood smears for malaria

Doctors use thick and thin blood smears to determine whether you have malaria. If one test is negative and no parasites are found, you will have repeated blood smears every 8 hours for a couple of days to confirm that there is no malaria infection.

Blood smears are taken most often from a finger prick. Thick and thin blood smears will let doctors know the percentage of red blood cells that are infected (parasite density) and what type of parasites are present.

• A thick blood smear is a drop of blood on a glass slide. Thick blood smears are most useful for detecting the presence of parasites because they examine a larger sample of blood. (Often there are few parasites in the blood at the time the test is done.)
• A thin blood smear is a drop of blood that is spread across a large area of the slide. Thin blood smears helps doctors discover what species of malaria is causing the infection.

To date, microscopic examination of thick and thin blood smears is the easiest and most reliable test for malaria.

Results of thick and thin blood smears may show:

Normal

No parasites are present in red blood cells. Your doctor will repeat the test every 8 hours for 1 or 2 days if he or she still suspects that you have malaria.

Abnormal

Parasites are present in red blood cells. The infecting species of Plasmodium is identified. Also, the percentage of red blood cells infected by the Plasmodium parasite (density) is determined.

Treatment may vary depending on the:

• Species of Plasmodium present. Malaria caused by P. falciparum is more serious than other types and may be treated differently.
• Percentage of red blood cells infected (parasite density), not the number of parasites. If a large percentage of blood cells is infected, medication may be given directly into a vein (intravenously, or IV) instead of by mouth (orally).

Blood smears are the most reliable tests for malaria. You may want to ask whether a thick or thin blood smear, or both, is planned. A thin blood smear will identify the species of the malaria parasite. This information is important to prevent or anticipate life-threatening complications if P. falciparum is the source of infection.

Complete the medical test information form (PDF)(What is a PDF document?) to help you prepare for this test.