Dengue Fever

Dr. Bhaskar Shenoy, Dr. Archana M
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Supportive Lab Investigations
CBC:
• Leucopenia is common initially, of which neutrophils usually predominate. Towards the end of febrile phase there is drop in total white cells and neutrophils. A relative lymphocytosis with more than 15% atypical lymphocytes is observed at end of febrile phase and at early stage of shock. Absence of leucopenia and a relative low percentage of typical lymphocytes predict severe dengue illness. (1,8)
• Leucopenia in combination with a positive tourniquet test in a dengue endemic area has a positive predictive value of 70-80%. (1)
• Thrombocytopenia below 1 lakh/cumm is found between third and eighth day of illness. (8) Parallel drop in platelets and progressive leucopenia precedes plasma leakage. (5) By ninth day platelets increase and recovers within 2 weeks. (5)
• Hematocrit rises by 10% due to dehydration. If it rises by 20% or more it is objective evidence of increased vascular permeability and leakage of plasma. (1,8)

LIVER FUNCTION TESTS
• Mild elevation in transaminase levels may be seen
• Low albumin is a sign of hemoconcentration (1,8)

COAGULATION STUDIES
• Partial thromboplastin time and prothrombin time (PT) are prolonged in about one-half and one-third of DHF case respectively. Delayed PT is a risk factor for DSS.
• Low fibrinogen and elevated fibrin degradation product levels are signs of disseminated intravascular coagulation.
• In severe cases with marked liver dysfunction, reduction is observed in vitamin dependent prothrombin family such as factors V, VII, IX and X. (1,8)

METABOLIC TESTS
• Hyponatremia, metabolic acidosis and elevated BUN are observed in patients with shock. (8)

Table 1.Advantages and disadvantages of confirmatory laboratory tests for dengue fever (1)
Tests Advantages Disadvantages
Viral antigen detection Easy to perform; rapid tests can be used in the field and provide results in a few hours; early diagnosis is possible, which may affect management. Specificity is nearly 100%. Sensitivity for primary dengue in the first 4 days is 90% and for secondary dengue is 70%.(8) May be as sensitive as viral nucleic acid detection; however, does not identify serotype
Viral nucleic acid detection Most sensitive and specific test available, especially in early infection; early diagnosis is possible (virus can be quantified in 1.5 hrs), which may affect management; can identify all 4 serotype (4) Expensive, requires laboratory facilities and expertise, not rapid (takes 24-48 hours), cannot differentiate between primary and secondary infection, potential for false positive results owing to contamination. Done when there is diagnostic problem, used as more of a research tool (8)
Serology Inexpensive, easy to perform, more readily available in dengue endemic areas, can distinguish between primary and secondary infection (that is, IgM to IgG ratio <1.2 suggests secondary infection) Lower specificity than other tests; requires two serum samples


Table 2: Interpretation of NS 1 and dengue serology (8)
NS1 antigen IgM IgG Interpretation
Positive Negative Negative Early(<4 days)
Negative/Positive Positive Negative Primary
Negative Positive Positive low titres Current/Recent
Negative/Positive Positive Positive high titres Secondary
Negative Positive Positive high titres Secondary
Negative Negative Positive low titres Past infection


Laboratory criteria for diagnosis of dengue haemorrhagic fever or dengue shock syndrome
• Rapidly developing, severe thrombocytopenia
• Decreased total white cell count and neutrophils and changing neutrophil to lymphocyte ratio
• Increased haematocrit (20% increase from baseline is objective evidence of plasma leakage)
• Hypoalbuminaemia (serum albumin <35g/L suggests plasma leakage)
• Increased liver function test results(aspartate aminotransferases:alanine aminotransferase>2)

• Chest radiograph- A lateral decubitus chest radiograph of right side of the chest detects undetectable pleural effusion in early phase of plasma leakage. Bilateral pleural effusions are common in patient with DSS. (1,7,8)
• Ultrasonography is useful to detect the presence of ascites, pleural effusion and gallbladder wall edema, which are common during critical phase and correlate with disease severity. Gall bladder wall edema precedes ascites and effusions hence helpful as early predictor of outcome. But gallbladder thickness may be increased in other infections and in postprandial states. (7,1)


References
Dengue Fever Dengue Fever 06/28/2016
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