TABLE 4

Fecal fixatives used in diagnostic parasitology, i.e., with intestinal tract specimens, and testing compatibility with the fixativef

FixativeConcentratePermanent-stain smear (trichrome, iron-hematoxylin, special stains for coccidia and Microsporidia)Immunoassays for Giardia lamblia and Cryptosporidium spp.Comment(s)
5%, 10% formalinYesNoYesConcentrations and IAs (EIA, FA, Rapids)
5%, 10% buffered formalinYesNoYesConcentrations and IAs (EIA, FA, Rapids)
MIFYesPolychrome IV stainNDNo published data
SAFYesIron-hematoxylin (best)YesConcentrations, permanent stains, and IAs (EIA, FA, Rapids)
Schaudinn's (Hg base), no PVAaRareYesNoPermanent stains; Hg interferes with IAs; primarily used with fresh stool specimens (no fixative collection vials)
Schaudinn's (Hg base) + PVAaRareYesNoPermanent stains; Hg and PVA interfere with IAs; considered the gold standard fixative for permanent stains
Schaudinn's (Cu base) + PVAbRareYesNoPermanent stains; PVA interferes with IAs; stains not as good as with Schaudinn's fixative using Hg or Zn
Schaudinn's (Zn base) + PVAcRareYesNoPermanent stains; PVA interferes with IAs; this is the same fixative as Total-Fix without PVA (see below)
EcoFix (PVA)dRareYesNoPermanent stains; PVA interferes with IAs; works best with EcoStain, Wheatley's trichrome (2nd best)
Universal-fixativee Total-FixYesYesYesNo formalin, no mercury, no PVA; concentrations, permanent stains, special stains, fecal IAs, PCR
  • a These two fixatives use the mercuric chloride base in the Schaudinn's fixative; this formulation is still considered to be the gold standard against which all other fixatives are evaluated (organism morphology after permanent staining).

  • b This modification uses a copper sulfate base rather than mercuric chloride. The morphology of stained organisms is not as good as with Hg or Zn.

  • c This modification (proprietary formula) uses a zinc base rather than mercuric chloride and works well with both trichrome and iron-hematoxylin.

  • d This fixative uses a combination of ingredients but is prepared from a proprietary formula (contains PVA).

  • e This modification uses a combination of ingredients (including zinc) but is prepared from a proprietary formula. The aim is to provide a universal fixative that can be used with fecal concentrations and with permanent-stain smears and available immunoassays for Giardia lamblia, Cryptosporidium spp., and Entamoeba histolytica (or the Entamoeba histolytica/E. dispar group). However, currently, fecal immunoassays for the Entamoeba histolytica/E. dispar group or Entamoeba histolytica (true pathogen) require fresh or frozen specimens; testing can also be performed from stool submitted in Cary-Blair transport medium. It is important to remember that immunoassays are performed on direct samples from the fixed stool, not on the sediment after concentration methods have been used; otherwise, antigens, especially soluble antigens, may be diluted and reduce assay sensitivity. However, centrifuged material can enhance the sensitivity of the direct fluorescent assay procedure and permanent stains, since the actual organisms (Cryptosporidium, Giardia) are seen microscopically.

  • f IA, immunoassay; Cu, copper; EIA, enzyme immunoassay; FA, fluorescent antibody; Hg, mercury; MIF, merthiolate-iodine-formalin fixative; ND, no data; PVA, polyvinyl alcohol; Rapids, cartridge format membrane flow IAs; SAF, sodium acetate-acetic acid-formalin; Zn, zinc.