Schistosomiasis laboratory findings
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] ; Associate Editor(s)-in-Chief: Aditya Ganti M.B.B.S. [2]
Overview
Methods for diagnosing schistosomiasis include visualization of Schistosoma eggs in stool, urine, and crushed biopsy tissues. serologic assays and urinary antigen testing have also been used. Laboratory findings consistent with the diagnosis of schistosomiasis include detection of circulating antibodies to schistosomes and schistosomal antigen in serum.
Laboratory Findings
Visualization of Schistosoma eggs in stool, urine, and crushed biopsy tissues is diagnostic of schistosomiasis. Laboratory findings consistent with the diagnosis of schistosomiasis include detection of circulating antibodies to schistosomes and schistosomal antigen in serum. Diagnostic tests for schistosomiasis include the following:[1][2][3]
- Microscopic examination of stool
- Urine testing for schistosome eggs
- Serologic testing
- Schistosomal antigen testing (urine or serum)
- Microscopic examination of tissue
- PCR to detect schistosomal DNA
Microscopic examination of stool
- The classic and most commonly used method for identification of schistosome eggs in stool is a modified Kato-Katz thick smear.[4]
- Testing should be done on formed stool, as schistosomiasis typically does not cause diarrhea.
- Several areas of a stool specimen should be evaluated independently, as eggs are not deposited uniformly throughout. In addition, eggs are not deposited uniformly throughout the day, and, thus, three different stool specimens should be evaluated.
- One to 99 eggs/g is suggestive of mild infection, 100 to 299 eggs/g indicate moderate infection, and more than 300 eggs/g are indicative of high-intensity infection.
- Kato Katz smears are not sufficiently sensitive for detection of low-intensity infections. Other techniques may be superior but often are unavailable or more difficult to use.
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Other Methods
Formalin-ethyl acetate sedimentation
- Five grammes of stool is mixed, strained, diluted with normal saline solution and centrifuged
- The sediment is collected and treated with formalin-ethyl acetate and subsequently used for slide preparation
- A single formalin-ethyl acetate sedimentation test is not as sensitive for detection of low-intensity infection as multiple Kato-Katz smears
Urine testing for schistosome eggs
- The classic method used for identification of S.haematobium eggs is filter concentration of a urine sample collected over 4 hours (ending around noon) into a jug with formalin preservative
- 10 mL of urine is filtered through a 12-μm pore membrane that traps the eggs, and the membrane surface then is examined under a microscope.
- Standard microscopic urinalysis will not identify low-intensity Schistosoma infections.
- Each separate microscopic urinalysis has a sensitivity of 55% to 62% for detection of low-intensity infection; therefore, at least three different urine samples need to be evaluated to achieve diagnostic accuracy.
Schistosomal antigen testing (urine or serum)
- Urine sample is taken for measurement of circulating cathodic antigen released by schistosomes or serum sample for measurement of both circulating cathodic and anodic antigen.[5]
- Identifies active infection rather than past infection
- May not be sufficiently sensitive for detection of low-intensity infection
Serologic testing
- Serologic testing help in detection of Schistosoma-specific antibodies in serum. These tests include:
- Enzyme-linked immunosorbent assay
- Indirect hemagglutination assay
- Indirect immunofluorescent antibody testing
- More useful for evaluating recent travelers than immigrants, as it is not possible to distinguish between active infection and past infection.
- Due to the long life of schistosomes, positive test results cannot be discounted simply because exposure was historically distant.
- Sensitivity is highest when the assay is targeted to the suspected species (S.mansoni, S.japonicum, or S.haematobium)
Biopsy of tissue
- A biopsy specimen is obtained from the rectum during anoscopy, genital tissues, or the urinary bladder wall during cystoscopy and then crushed and examined under a microscope
- S.mansoni and S.japonicum eggs can be identified in crushed random rectal biopsy specimens.
- S.haematobium eggs can be identified in crushed biopsy specimens from genital tissues or the urinary bladder wall
- Sensitivity of microscopic analysis of six crushed rectal biopsies is similar to that of two Kato-Katz thick smears.
- Liver biopsy is notoriously insensitive for diagnosis of schistosomiasis; a negative liver biopsy result does not exclude infection
- Standard sectioned intestinal biopsies are not sufficiently sensitive for diagnosis of intestinal schistosomiasis
PCR to detect schistosomal DNA
- Gene amplification technique used to detect schistosomal DNA.
Other laboratory tests
Other diagnostic tests that are helpful in diagnosis of schistosomiasis include:
- Urinalysis, including dipstick testing and microscopic analysis for leukocytes, erythrocytes, and casts.
- If obstruction is causing a urinary tract infection, leukocyte esterase or nitrites may be present.
- Erythrocytes are seen in the urine of patients with glomerulonephritis.
- Urinary casts, which are aggregates of protein, blood cells, tubular epithelial cell constituents, or all three, develop secondary to urinary stasis in renal tubules and significant proteinuria.
- Measurement of blood urea nitrogen (BUN) and serum creatinine to test renal function.
- Liver function tests
- AST and ALT levels usually remain normal, even in patients with hepatosplenic disease.
- Albumin levels may be low due to malnutrition or nephrotic forms of schistosomiasis.
- Complete blood count (CBC)
- Anemia may be seen in patients with chronic blood loss due to intestinal or urinary schistosomiasis and in those with glomerular disease.
- Eosinophilia may be prominent early in the disease course but may be minimal in patients with longstanding disease.
References
- ↑ Ibironke OA, Phillips AE, Garba A, Lamine SM, Shiff C (2011). "Diagnosis of Schistosoma haematobium by detection of specific DNA fragments from filtered urine samples". Am. J. Trop. Med. Hyg. 84 (6): 998–1001. doi:10.4269/ajtmh.2011.10-0691. PMC 3110375. PMID 21633040.
- ↑ "CDC - DPDx - Schistosomiasis Infection - Laboratory Diagnosis".
- ↑ Weerakoon KG, Gobert GN, Cai P, McManus DP (2015). "Advances in the Diagnosis of Human Schistosomiasis". Clin. Microbiol. Rev. 28 (4): 939–67. doi:10.1128/CMR.00137-14. PMC 4548261. PMID 26224883.
- ↑ Tarafder MR, Carabin H, Joseph L, Balolong E, Olveda R, McGarvey ST (2010). "Estimating the sensitivity and specificity of Kato-Katz stool examination technique for detection of hookworms, Ascaris lumbricoides and Trichuris trichiura infections in humans in the absence of a 'gold standard'". Int. J. Parasitol. 40 (4): 399–404. doi:10.1016/j.ijpara.2009.09.003. PMC 2829363. PMID 19772859.
- ↑ van Etten L, Folman CC, Eggelte TA, Kremsner PG, Deelder AM (1994). "Rapid diagnosis of schistosomiasis by antigen detection in urine with a reagent strip". J. Clin. Microbiol. 32 (10): 2404–6. PMC 264074. PMID 7814474.