Ova and Parasite (O&P) Examinations

Introduction | Macroscopic | Microscopic |

Microscopic Examination - Ova and Parasite Examination

Introduction | Direct Wet Smear | Concentration | Permanent Stained Smear

Concentration

Procedure Limitations

Results obtained with wet smears (direct wet smears or concentrated specimens) should usually be confirmed by permanent stained smears. Some protozoa are very small and difficult to identify as to species with just the direct wet smears. Also, special stains are sometimes necessary for organism identification (Cryptosporidium parvum, microsporidia).
Confirmation is particularly important in the case of E. histolytica/E. dispar versus E. coli.
Certain organisms (G. lamblia, hookworm eggs, and occasionally Trichuris eggs) may not concentrate as well from PVA-preserved specimens as they do from those preserved in formalin. However, if there are enough G. lamblia organisms present to concentrate from formalin, then PVA should contain enough for detection on the permanent stained smear. In clinically important infections, the number of helminth eggs present would ensure detection regardless of the type of preservative used. Also, the morphology of S. stercoralis larvae is not as clear from PVA as from specimens fixed in formalin.
For unknown reasons, I. belli oocysts are routinely missed in the concentrate sediment when concentrated from PVA-preserved specimens. The oocysts would be found if the same specimen was preserved in formalin rather than PVA.
In past publications, recommended centrifugation times have not taken into account potential problems with the recovery of Cryptosporidium oocysts. There is anecdotal evidence and personal experience (Lynne Garcia) to strongly indicate that Cryptosporidium oocysts may be missed unless the centrifugation speed is 500 x g for for a minimum of 10 min. Higher numbers of Cryptosporidium oocysts and microsporidian spores were found in concentrate sediment after the previously recommended centrifugation speed and time were revised to 500 x g for a minimum of 10 min.
Adequate centrifugation time and speed have become very important for recovery of microsporidial spores. In some of the earlier publications, use of unspun material was recommended. However, we have found that centrifugation for 10 min at 500 X g definitely increases the number of microsporidial spores available for staining and subsequent examination.