Intestinal parasites: Entamoeba histolytica

Faecal parasite diagnosis - UKAS accredited test

Faecal microscopy (including hot stool)

A concentration technique is routinely performed on all faecal samples for the presence of cysts, ova and larvae. Direct saline smears are examined on samples that are 24 hours old or less where trophozoite stages of protozoan parasites are suspected.

Sample requirement: Minimum of ¼ specimen pot of faecal sample. Ova, cysts and parasites may be passed intermittently therefore three samples may be required to be examined. Ova, cysts and parasites will diminish over time therefore it is more ideal for the sample to be less than 2 days old upon receipt at LSTM, however older stools will not be rejected.

Key factors affecting test: trophozoites may only survive for up to 24 hours in voided faecal samples

If Entamoeba histolytica trophozoites are suspected a “hot” stool must be examined.

Hot stool sample requirement: specimen must reach the laboratory within 30 minutes of sample being produced and must be marked as hot stool, this will then be examined with priority. Methanol fixed faecal smears can be made at requesting site if the sample can not arrive at CDPL within 30 minutes, these smears can then be sent to CDPL for staining and examination.

Faecal Antigen detection

Entamoeba histolytica adhesin ELISA

To differentiate between cysts of E.histolytica/dispar as seen by faecal microscopy.

Sample requirement: samples must not be bloody. The test must be performed on FRESH (< 48 hour old) NOT FIXED specimens OR unpreserved specimens should be kept at 2-8°C and tested within 48 hours of defaecation. If the test cannot be performed within 48hours store sample at -20°C or lower until tested. Do NOT send preserved specimens. Minimum of ¼ pot of faecal specimen.

Key factors affecting test: test not validated on samples older than specified.

Amoebiasis – examination of cyst aspirate/pus samples

UKAS accredited test          

Specimen requirement: Requests for Entamoeba histolytica in samples of pus from liver abscesses should either be examined within one hour of collection or fresh THIN smears of pus should be made (minimum of 3), fixed when dry (1 minute in methanol) and sent for staining together with pus sample.

Key factors affecting test: About 30 minutes after aspiration of cyst pus/fluid E.histolytica trophozoites become indistinguishable from macrophages.