Explanation

Malaria parasite (thick blood smear): what is measured?

Malaria is caused by single-celled parasites of the genus Plasmodium, which enter the blood through the bite of an infected mosquito and invade red blood cells there. In this test, a laboratory specialist examines blood under the microscope. In a thick blood smear, a layer of blood in which the red blood cells have been dissolved, parasites are searched for; in a thin blood smear, the specialist looks at which species it is and what share of the red blood cells is affected. According to the LCI guideline (RIVM) on malaria, this is the standard test.

What can it detect?

A current malaria infection
Which Plasmodium species it is, which determines the severity and the treatment
How many of the red blood cells are affected (parasitemia), which is taken into account when assessing the severity

Result

What does the result mean?

Positive result

Malaria parasites were seen, which fits a current infection. Malaria can quickly become serious, especially with Plasmodium falciparum: that form can become life-threatening within a few days. With Plasmodium vivax, ovale and malariae, the illness is usually milder, but those forms also need to be treated. Symptoms that fit are fever, chills, headache, muscle pain, nausea and a flu-like feeling. Malaria can be treated well with medicines, provided treatment starts quickly.

Negative result

No parasites were seen. This does not rule out malaria. Early in the illness there may still be too few parasites in the blood, and with Plasmodium falciparum, parasites can temporarily stay in small blood vessels, so that little can be seen in the sample. If the suspicion persists, the test is therefore repeated, usually within one to two days. Malaria tablets or treatment that has already started can also temporarily bring the number of parasites below the detection limit.

After an abnormal result

Fever during or after a stay in an area where malaria occurs is an emergency and should be assessed by a doctor the same day; do not wait for the result of a self-requested test. With a negative result and ongoing symptoms, the test is repeated. With a positive result, the doctor decides, based on the species and the share of affected red blood cells, which treatment is needed and whether hospital admission is needed; with Plasmodium vivax and ovale, a follow-up treatment against the dormant liver stages is also given. Malaria is notifiable (group C): a confirmed infection is reported to the GGD (municipal public health service) within one working day.

Reliability

How reliable is the test?

According to the LCI guideline (RIVM) on malaria, the thick blood smear is the standard test: it shows the parasite itself and also shows the species and the share of affected red blood cells. How small an infection can still be detected depends strongly on the experience of the person assessing the smear. One negative result does not rule out malaria; if the suspicion persists, the test should be repeated. The test is intended for people with symptoms, not as a check afterwards in people without symptoms.

What can affect the result?

Early in the illness, with few parasites in the blood, the test can be negative; repeating it is then needed.
Malaria tablets or treatment that has already started can temporarily lower the number of parasites.
With Plasmodium falciparum, parasites can stay in small blood vessels, so that little can be seen in the blood sample while the infection is serious.
The assessment is manual work under the microscope and requires experience; the result therefore partly depends on the laboratory.

Who it is for

Who is the test useful for?

Useful for

Intended for people with fever or flu-like symptoms during or after a stay in an area where malaria occurs. Symptoms usually start about a week to a month after the mosquito bite, but with some species they can appear only months to years later. For people without symptoms and without a stay in a malaria area, the test has no value.

How common is it?

Between 2014 and 2024, 72 to 343 cases of malaria were reported each year in the Netherlands; in 2023 there were 249 and in 2024 there were 217. About 90 in 100 reports concern an infection acquired in Africa, often during a visit to family or friends. Malaria is not transmitted in the Netherlands itself.

RIVM/LCI guideline on malaria (reports 2014 to 2024)

Drawbacks and risks

A negative result can give false reassurance: it does not rule out malaria, and with Plasmodium falciparum, delaying assessment can become life-threatening within a few days.
Requesting a test yourself and waiting for the result takes time, while fever after a stay in a malaria area requires immediate assessment by a doctor.

Get tested