What are the diagnostic tests for malaria in a patient with a suspected infection?

Diagnostic Tests for Malaria

For any patient with suspected malaria, perform both thick and thin blood smears with Giemsa stain alongside a rapid diagnostic test (RDT) immediately, and if the first test is negative but suspicion remains high, repeat testing every 12-24 hours for three consecutive days. 1

Primary Diagnostic Methods

Microscopy (Gold Standard)

• Thick and thin blood films stained with Giemsa or May-Grunwald-Giemsa remain the reference standard because they allow species identification, parasitemia quantification, and differentiation between sexual and asexual forms 1, 2
• Three sets of thick and thin blood smears examined over 72 hours are required to confidently exclude malaria in patients with ongoing suspicion 1, 3
• Thick films have sensitivity equivalent to RDTs when read by an expert microscopist, but blood films are essential for speciation and parasite counting 1
• Positive blood films (both thick and thin) should be sent to a reference laboratory for confirmation 1

Rapid Diagnostic Tests (RDTs)

• RDTs provide results within 15 minutes with sensitivity for P. falciparum ranging from 67.9% to 100% and specificity between 98.1% to 100% 1, 4
• For P. vivax, RDT sensitivity ranges from 66% to 91% with specificity of 98.1% to 100% 1
• RDTs should be used as initial screening but must not replace microscopy, which should be performed in parallel 1, 5
• RDTs are particularly valuable when expert microscopy is not immediately available 5

Molecular Testing (PCR/LAMP)

• PCR testing is available through CDC and reference laboratories for confirmatory diagnosis when microscopy results are atypical or species cannot be determined 1
• PCR amplifies the small subunit rRNA gene and provides definitive parasite and species diagnosis 1
• LAMP (Loop-mediated isothermal amplification) has sensitivity of 96.4% to 100% and specificity of 93.8% to 100% 1
• Molecular methods are 10-100 times more sensitive than microscopy but are generally limited to specialized laboratories 4

Essential Supporting Laboratory Tests

Complete Blood Count

• Thrombocytopenia (<150,000/mL) occurs in 70-79% of malaria cases regardless of species and is the most frequent laboratory finding 1, 6
• Screen all thrombocytopenic samples with platelet counts <100,000/mL for malaria to avoid missed diagnoses 1
• Severe anemia is defined as hemoglobin <7 g/dL or hematocrit <20% with parasite count >10,000/mL 3
• Lymphopenia is common in malaria, while leukocytosis may indicate secondary bacterial infection or severe disease 3
• Malaria pigment in neutrophils and monocytes provides diagnostic clues even when blood films are initially negative 3

Metabolic and Organ Function Tests

• Blood glucose monitoring is critical, with hypoglycemia defined as <40 mg/dL (2.2 mmol/L) 3
• Arterial blood gas or venous lactate measurement: acidosis with pH <7.35 or plasma bicarbonate <15 mmol/L, or hyperlactatemia with venous plasma lactate >5 mmol/L indicates severe disease 3
• Serum creatinine: acute renal failure defined as >3 mg/dL (>265 mmol/L) 3
• Hyperbilirubinemia (>3 mg/dL or >50 mmol/L) with parasite count >100,000/mL indicates severe malaria 1, 3

Additional Diagnostic Studies

• Blood cultures (two sets) should be obtained before antibiotics if secondary bacterial infection is suspected, particularly with leukocytosis 1, 3
• Chest radiography if pulmonary edema or ARDS is suspected 3
• Urinalysis may show proteinuria and hematuria (in leptospirosis differential) or hemoglobinuria in severe malaria 1

Critical Testing Protocols

Timing and Frequency

• Perform malaria testing in all patients who have visited a tropical country within 1 year of presentation 1
• If initial microscopy and RDT are negative but suspicion remains high, repeat testing daily for three consecutive days 1, 5
• In severe cases, monitor parasitemia every 12 hours until decline is detected, then every 24 hours until negative 3, 4

Special Considerations

• G6PD testing must be performed before primaquine administration to prevent hemolysis 3, 4
• In immunocompromised patients, test even if prophylaxis was taken or if the patient is from an endemic area 3
• Consider broader infectious workup including CSF examination if neurological symptoms are present 3

Common Pitfalls to Avoid

• Do not delay treatment while awaiting blood film results if cerebral malaria is suspected - start treatment immediately and obtain specialist advice 3, 4
• A single negative blood smear does not rule out malaria; three negative smears at 12-hour intervals are necessary to exclude diagnosis 6
• Do not miss hypoglycemia, especially in pregnant women receiving IV quinine 3, 4
• Normal white blood cell count does not rule out malaria 3
• The delay in laboratory diagnosis (greater than 3 hours) should not prevent initiation of empirical antimalarial treatment if the probability of malaria is high 5
• When diagnoses are made by locally trained microscopists, send a random sample of both positive and negative slides to a reference laboratory for quality control verification 4

Documentation Requirements

• Document complete travel history on all request forms, including specific locations visited, dates of travel, dates of symptom onset, and risk activities undertaken 1
• Pathogen-specific request forms are required by reference laboratories for some infections and are available through public health websites 1