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Parasitemia and tissue parasites

15 min

Skills you'll build3 skills
  • Calculate asexual parasitemia from infected and total red cells
  • Calculate parasite density using the measured leukocyte count
  • Select a specimen and method for a suspected tissue protozoal infection

Try first

Try first

On a thin film, 2,000 red cells are counted. Of these, 52 contain malaria rings, and 8 of those 52 contain two rings each. What is the percent parasitemia?

Keep going to see the reasoning.

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Percent parasitemia on the thin film

Every smear-positive malaria case is reported with a quantitative parasitemia.1,2 On the thin film:

Percent parasitemia = infected red cells ÷ red cells counted × 100

Count 500 to 2,000 red cells in the monolayer, where cells touch without overlapping, following the laboratory's procedure. A red cell containing several asexual parasites is counted once. Cells containing only gametocytes are excluded from the asexual count.1

Parasite density from the thick film

When percent parasitemia cannot be counted reliably, the thick film gives a density:1

Parasites/µL = asexual parasites counted ÷ leukocytes counted × patient's leukocytes/µL

A common endpoint is 500 parasites or 1,000 leukocytes, whichever comes first. The patient's measured leukocyte count is used whenever one is available. If the procedure substitutes 8,000 leukocytes/µL, the report states that assumption.1 During treatment, quantitation is repeated at the interval the clinical protocol sets, using the same method.1,2

Toxoplasma testing by specimen

For Toxoplasma gondii, the specimen depends on the question being asked.3,4

QuestionSpecimen and test
Has this adult been infected, and when?Serum IgG and IgM, with a positive IgM confirmed by a toxoplasmosis reference laboratory
Is the fetus infected?PCR of amniotic fluid
Is the newborn infected?Reference-laboratory IgM and IgA, compared with the mother's antibody, and serial infant IgG
Is there encephalitis in an immunocompromised patient?CSF PCR, and histology or PCR of tissue

Toxoplasma IgM can last 18 months or longer after infection and can be falsely positive. One positive IgM therefore cannot date an infection.3 Maternal IgG crosses the placenta, so a newborn's IgG alone cannot be distinguished from the mother's antibody. CSF PCR is specific, and it misses some cases, especially after treatment begins.4

Follow the bench demonstration

Sources for this step
References
  1. Centers for Disease Control and Prevention. DPDx: blood specimens. Accessed September 27, 2026. https://www.cdc.gov/dpdx/diagnosticprocedures/blood/index.html
  2. Centers for Disease Control and Prevention. Evaluation and diagnosis of malaria. Accessed September 27, 2026. https://www.cdc.gov/malaria/hcp/clinical-guidance/evaluation-diagnosis.html
  3. Centers for Disease Control and Prevention. DPDx: toxoplasmosis. Reviewed May 5, 2025. Accessed September 27, 2026. https://www.cdc.gov/dpdx/toxoplasmosis/index.html
  4. Miller JM, Binnicker MJ, Campbell S, et al. Guide to utilization of the microbiology laboratory for diagnosis of infectious diseases: 2024 update by the Infectious Diseases Society of America and the American Society for Microbiology. Clin Infect Dis. 2024;ciae104. doi:10.1093/cid/ciae104

Watch one

Read each decision and its explanation.

A thin film from a traveler with P. falciparum malaria is counted in the monolayer. The laboratory professional counts 1,200 red cells. Of these, 30 hold asexual rings, 5 of them with two rings. Another 4 cells hold only crescent gametocytes. What is the percent parasitemia?

  1. Confirm the area: the count was made in the monolayer.

    The feathered edge distorts cells, and thick areas overlap them.

  2. Set the numerator: 30 infected cells. The 5 doubly infected cells count once each.

    Percent parasitemia counts infected cells, regardless of how many parasites each contains.

  3. Exclude the 4 cells that contain gametocytes only.

    Gametocytes are recorded separately from the asexual count.

  4. Set the denominator: 1,200 red cells.

    The denominator is every red cell counted.

  5. Calculate: 30 ÷ 1,200 × 100 = 2.5%.

    The ratio multiplied by 100 gives the percentage.

The asexual parasitemia is 2.5%. Gametocytes are reported separately.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

On a thin film, 1,500 erythrocytes are counted. Of these, 45 contain asexual parasites, 6 of them containing two rings each, and 3 others contain only gametocytes. What is the reported percent parasitemia?

Correct. Percent parasitemia = 45 ÷ 1,500 × 100 = 3.0%. A cell with two rings is one infected cell, and gametocyte-bearing cells are left out of the asexual parasitemia.

Incorrect. This adds the 3 gametocyte-bearing cells: 48 ÷ 1,500 × 100. Gametocytes are recorded separately and excluded from the asexual count.

Incorrect. This counts parasites, 45 + 6 = 51. An erythrocyte with two rings is still one infected cell, so the numerator is 45.

Hint
  1. The numerator is the number of infected cells, counting each cell once.
  2. Decide which cells belong in an asexual parasitemia.

Review Parasitemia

Problem 2 of 3 · Hints available

Following the laboratory's procedure, 200 asexual parasites are counted against 500 leukocytes on a thick film. The patient's measured leukocyte count is 6,500/µL. What is the parasite density?

Incorrect. This divides by 1,000 leukocytes, the common counting endpoint. The count stopped at 500 leukocytes, so density = 200 ÷ 500 × 6,500 = 2,600 parasites/µL.

Correct. Density = 200 ÷ 500 × 6,500 = 2,600 parasites/µL. The measured leukocyte count is used whenever it is available, and a report based on the assumed 8,000/µL states that assumption.

Incorrect. This uses the assumed 8,000 leukocytes/µL. A measured count replaces the assumption: 200 ÷ 500 × 6,500 = 2,600 parasites/µL.

Hint
  1. Divide the parasites by the leukocytes counted with them.
  2. Then multiply by a leukocyte count, using this patient's measured count.

Review Parasitemia

Problem 3 of 3 · On your own

A thick film is counted until 1,000 leukocytes are reached. By then 350 asexual parasites have been counted. The patient's measured leukocyte count is 9,200/µL. What is the parasite density?

Show the answer

3,220 parasites/µL

Parasites/µL = 350 ÷ 1,000 × 9,200/µL = 3,220 parasites/µL.

Review Parasitemia

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Ana Sofía Washburn, 31, a pastry chef who tastes everything she cooks, is 14 weeks pregnant.
  • A prenatal screen at an outside clinic shows Toxoplasma IgG positive and IgM positive.
  • Her obstetrician calls the laboratory to ask what the results mean and what comes next.
Decision 1 of 3

What does the laboratory advise about the positive IgM?

Toxoplasma IgM can last 18 months or more and can be falsely positive. One positive IgM cannot date the infection.

The reference laboratory's panel, including avidity, can show whether the infection is likely recent.

A repeat of the same test cannot distinguish persistent IgM from recent IgM. The confirmatory panel can.

Review Specimen selection and preservation

Decision 2 of 3

The reference laboratory reports that infection during pregnancy is likely. The obstetrician asks how fetal infection can be tested. What specimen fits?

Maternal blood is the mother's specimen. It does not show whether the parasite reached the fetus.

Used maternal blood PCR to test for fetal toxoplasmosis

Fetal infection is diagnosed by PCR of amniotic fluid collected at 18 weeks of gestation or later and at least 4 weeks after the suspected maternal infection. Maternal blood PCR is not the routine test for fetal infection, and maternal serology shows only whether the mother was infected.

PCR of amniotic fluid detects fetal infection. The timing depends on gestational age and the estimated date of maternal infection.

Maternal serology shows the mother's infection. It does not show whether the fetus is infected.

Review Specimen selection and preservation

Decision 3 of 3

At birth the baby appears well, and the baby's Toxoplasma IgG is positive. What does that result show?

Maternal IgG crosses the placenta. A newborn's IgG can be entirely the mother's.

Many infected newborns appear well at birth. Appearance cannot exclude infection.

Infant IgM and IgA, compared with the mother's antibody, and IgG that persists or rises past the expected loss of maternal antibody show congenital infection.

Review Specimen selection and preservation

Each result depends on whose antibody or DNA is being measured. Maternal serum shows the mother's infection, amniotic fluid PCR shows fetal infection, and the newborn's IgG may be maternal antibody.

For reference

Sources checked