Method and Organism Discussions

Discussion #1B: Reporting Malaria

Interpretation and Reporting of Results

Detection of malaria parasites is a critical result, which must be reported immediately to the clinician. The report should include the species identification, if possible and stages seen. If identification to this level is not possible, an explicit statement that P. falciparum and P. knowlesi cannot be excluded is advisable. If the microscopic diagnosis is P. malariae, but the patient has traveled to southeast Asia it is important that the clinician knows that P. knowlesi cannot be excluded.

Because patients with high parasitemias (>3-5%) require intensive therapy, quantification of parasitemia should always be included. The determination of parasitemia with each set of positive blood films also allows the physician to track the results of therapy. This is typically performed after species identification and may be variously expressed as percent infected RBCs per100 RBCs, number of parasites per 200 leukocytes, or parasites/mm3. At high parasitemias, the direct counting of parasites and erythrocytes in a thin film will provide an accurate parasitemia. In low parasitemias and, of course, in thick smears, it may be necessary to count parasites/200 leukocytes and to calculate the percent parasitemia or the parasite density based on the patient’s complete blood count. A negative report may be accompanied by a reminder that a single negative set of smears does not exclude the diagnosis of malaria. Serial malaria smears may be performed to monitor therapy. Parasitemia normally resolves within 2-3 days following initiation of treatment with a drug to which the patient's strain is susceptible. Continued parasitemia on day 3 or failure of the parasitemia to decrease by 75% within the first 48 hours following treatment is an indication of drug resistance. However, gametocytes may continue to circulate for up to two weeks after successful cure, and their presence is not an indication that treatment has failed.

Additional Information Regarding Parasitemias

A. It is critical that the same reporting method be used consistently for every subsequent set of blood films so that the parasitemia can be tracked for decrease or possible increase, indicating resistance. For consistency, it is also recommended that the same individual who calculated the original result perform any additional calculations on subsequent blood films.
B. Remember that drug resistance may not become evident for several days; the parasitemia may even appear to be dropping before it begins to increase again.
C. It is important that any patient with P. falciparum infection be monitored; drug tolerance and resistance have also been reported for Plasmodium vivax infections.
D. It is critical to remember that mixed malarial infections occur, many of which include P. falciparum.

Reporting Results

A. Report Plasmodium spp., including the stage(s) seen (do not use abbreviations).

Examples:

Plasmodium falciparum rings and gametocytes (0.1%)

Plasmodium falciparum rings only, no gametocytes seen (0.002%)

Plasmodium vivax rings, trophozoites, schizonts, and gametocytes (0.03%)

B. Identification of Plasmodium species is crucial for proper clinical management of infected patients. Coinfections with multiple species are possible. Any laboratory providing malaria diagnoses should be able to identify Plasmodium vivax and Plasmodium ovale, even in the absence of Schüffner’s stippling.

C. If species identification from microscopic reading of blood films is uncertain, report to genus level and consider sending to a reference laboratory for further identification. PCR assays are highly sensitive and capable of providing species identification in cases of mixed infections. However, if blood films are sent to a reference laboratory for identification, the laboratory should always review smears prior to sending out. This eliminates the potential problem of a large lag time between when the blood was drawn and when the blood films were examined. REMEMBER: THESE ARE STAT PROCEDURES (ORDER, COLLECTION, PROCESSING, EXAMINATION, REPORTING).

Report Comments

Report comments can be extremely helpful in conveying information to the physician. Depending on the results of diagnostic testing, the following information can lead to improved patient care and clinical outcomes. The report is provided with the comment following.

1. No Parasites Seen: The submission of a single blood specimen will not rule out malaria; submit additional bloods every 4-6 hrs for 3 days if malaria remains a consideration.

Interpretation/Discussion: It is important to make sure the physician knows that examination of a single blood specimen will not rule out malaria.

2. Plasmodium spp. seen: Unable to rule out Plasmodium falciparum or Plasmodium knowlesi

Interpretation/Discussion: Since P. falciparum and P. knowlesi cause the most serious illness, it is important to let the physician know these species have NOT been ruled out.

3. Plasmodium spp., possible mixed infection: Unable to rule out Plasmodium falciparum or Plasmodium knowlesi.

Interpretation/Discussion: Since P. falciparum and P. knowlesi cause the most serious illness, it is important to let the physician know these species have NOT been ruled out.

4. Plasmodium malariae: Unable to rule out Plasmodium knowlesi.

Interpretation/Discussion: If the patient has traveled to the endemic area for P. knowlesi, it may be impossible to differentiate between P. malariae (band forms) and P. knowlesi.

5. Interpretation/Discussion: If the patient has traveled to the endemic area for P. knowlesi, it may be impossible to differentiate between P. falciparum (ring forms) and P. knowlesi..

6. Negative for parasites using automated hematology analyzer: Automated hematology analyzers will not detect low malaria parasitemias seen in immunologically naïve patients (travelers)

Interpretation/Discussion: In patients who have never been exposed to malaria (immunologically naïve), they will become symptomatic with extremely low parasitemias that will not be detected using automation (0.001 to 0.0001%)

7. Negative for malaria using the BinaxNOW rapid test: This result does not rule out the possibility of a malaria infection. Blood should be submitted for STAT thick and thin blood film preparation, examination, and reporting.

Interpretation/Discussion: The maximum sensitivity of the BinaxNOW rapid test occurs at 0.1% parasitemia. Patients (immunologically naïve travelers) may present to the emergency room or clinic with a parasitemia much lower than 0.1%, leading to a false negative report. Also, this rapid test is not designed to identify P. malariae, P. ovale, and P. knowlesi; the results are most clinically relevant for P. falciparum and P. vivax. The BinaxNOW is FDA cleared for use within the United States.

TABLE 1 PARASITEMIA DETERMINED FROM CONVENTIONAL LIGHT MICROSCOPY: CLINICAL CORRELATION AND INTERPRETATION

Parasitemia (%)	No. of Parasites/µl	Clinical Correlation
0.0001-0.0004	5-20	Number of organisms required for positive thick film (sensitivity). Examination of 100 thick-blood-film (TBF) fields (0.25 µl) may miss up to 20% of infections (sensitivity 80-90%); at least 300 fields should be examined before reporting a negative result. Examination of 100 thin-blood-film fields (THBF) (0.005 µl); at least 300 fields should be examined before reporting a negative result. Both thick and thin blood films should be examined for every specimen submitted for a suspect malaria case (report final results using 100x oil immersion objective). BinaxNOW® rapid lateral flow method (dipstick) (0-100 = 53.9% sensitivity for P. falciparum). BinaxNOW® rapid lateral flow method (dipstick) (0-100 = 6.2% sensitivity for P. vivax). One set (TBF + THBF) of negative blood films does not rule out a malaria infection.
0.002	100	Patients may be symptomatic below this level, particularly if they are immunologically naïve (no prior exposure to malaria, e.g., travelers). BinaxNOW® rapid lateral flow method (dipstick) (100-500 = 89.2% sensitivity for P. falciparum). BinaxNOW® rapid lateral flow method (dipstick) (100-500 = 23.6% sensitivity for P. vivax).
0.02	1,000	Level often seen in travelers (immunologically naïve) – results may also be lower than this. BinaxNOW® rapid lateral flow method (dipstick) (1000-5000 = 99.2% sensitivity for P. falciparum) (500-1000 = 92.6% sensitivity for P. falciparum). BinaxNOW® rapid lateral flow method (dipstick) (1000-5000 = 81.0% sensitivity for P. vivax) (500-1000 = 47.4% sensitivity for P. vivax).
0.1	5,000	BinaxNOW® rapid lateral flow method (dipstick) (>5000 = 99.7% sensitivity for P. falciparum). BinaxNOW® rapid lateral flow method (dipstick) (>5000 = 93.5% sensitivity for P. vivax).
0.2	10,000	Level above which immune patients will exhibit symptoms.
2	100,000	Maximum parasitemia of P. vivax and P. ovale (which infect young RBCs only).
2-5	100,000 – 250,000	Hyperparasitemia, severe malaria; increased mortality.
10	500,000	Exchange transfusion may be considered; high mortality.

^aThe BinaxNOW® malaria test (Alere) is FDA approved. The test detects antigen (viable and non-viable) organisms, including gametocytes and sequestered P. falciparum parasites. Performance depends on antigen load and may not directly correlate with microscopy. Positive rheumatoid factor titers may give false positives. The pan malarial test line is capable of detecting all five species. However, insufficient data is present to support claims for the detection of P. malariae, P. ovale, or P. knowlesi. Claims are made for P. falciparum and P. vivax only. The test is not intended for use in screening asymptomatic populations.

^bWorld Health Organization criteria for severe malaria are parasitemia of >10,000/µl and severe anemia (hemoglobin, <5 g/liter). Prognosis is poor if >20% of parasites are pigment-containing trophozoites and schizonts and/or if >5% of neutrophils contain visible pigment.

References

Garcia, L.S. 2016. Diagnostic Medical Parasitology, 6^th ed. ASM Press, Washington, DC, 1388 pp.

Garcia, LS, 2021. Practical Guide to Diagnostic Medical Parasitology, 3^rd ed., ASM Press, Washington, DC