Preservative

Concentration

Permanent stained smear, Wheatley’s trichrome, iron hematoxylin, special stains/coccidia and microsporidia

Immunoassays^a (Giardia lamblia (G. duodenalis, G. intestinalis), Cryptosporidium spp.)

Comments

5% or 10% formalin

Yes

No

Yes

EIA, FA, cartridge

5% or 10% buffered formalin

Yes

No

Yes

EIA, FA, cartridge

MIF

Yes

Polychrome IV stain

ND

No published data

SAF

Yes

Iron hematoxylin (best) or Trichrome (modified for SAF)

Yes

EIA, FA, cartridge

Schaudinn’s fixative^b (Mercury base) with or without PVA

Yes, but rarely used

Trichrome or iron hematoxylin

No

PVA interferes with immunoassays

Modified Schaudinn’s^c (Copper base) with PVA

Yes, but rarely used

Trichrome or iron hematoxylin

No

PVA interferes with immunoassays

Modified Schaudinn’s^d (Zinc base) with PVA

Yes

Trichrome or iron hematoxylin

No

PVA interferes with immunoassays

Single-vial systems^e (with PVA)(EcoFIX)(Meridian Bioscience)

Rare

Trichrome or iron hematoxylin; works best with EcoStain (Meridian Bioscience)

Some, but not all (No if contains PVA)

Check with the manufacturer

Universal Fixative^f (Single vial system)(TOTAL-FIX™)(Medical Chemical Corporation)

Yes

Trichrome or iron hematoxylin

Yes

Contain no mercury, no formalin, no PVA; does not require albumin for adhesion (no PVA, no albumin)

^a Fecal immunoassays for the Entamoeba histolytica/E. dispar complex or group or the true pathogen, Entamoeba histolytica require fresh or frozen specimens; in some cases, Cary-Blair may be acceptable – check with the manufacturer).

^b These two fixatives use the mercuric chloride base in the Schaudinn’s fluid; this formulation is still considered to be the "gold standard", against which all other fixatives are evaluated (organism morphology after permanent staining). Additional fixatives prepared with non-mercuric chloride-based compounds are used; however, the overall organism morphology is not as good (zinc-based options much better than copper).

^c This modification uses a copper sulfate base rather than mercuric chloride; organism morphology is marginal.

^d This modification uses a zinc base rather than mercuric chloride and works well with both trichrome and iron-hematoxylin stains.

^e These modifications use a combination of ingredients (including zinc), but are prepared from proprietary formulas. The aim is to provide a fixative that can be used for the fecal concentration, permanent stained smear, and available immunoassays for Giardia lamblia (G. duodenalis, G. intestinalis), Cryptosporidium spp.; Entamoeba histolytica (or the Entamoeba histolytica/E. dispar group)

^f Universal fixatives contain no mercury, no formalin, and no PVA. The complete O&P (concentration, permanent stained smear), fecal immunoassays for Giardia lamblia (G. duodenalis, G. intestinalis) and Cryptosporidium spp., and special stains for the Apicomplexa, coccidia, and microsporidia can be performed using these options. Fecal immunoassays for the Entamoeba histolytica/E. dispar group or the true pathogen, Entamoeba histolytica require fresh or frozen specimens; in some cases, Cary-Blair may be acceptable – check with the manufacturer. An example would be: TOTAL-FIX (Medical-Chemical Corporation, Torrance, CA). One Universal Fixative (TOTAL-FIX, Medical Chemical Corporation, Torrance, CA) has also been found to be compatible with a number of molecular test options, including PCR.

EIA, enzyme immunoassay; FA, fluorescent antibody; MIF, merthiolate-iodine-formalin; ND, no data; PCR, polymerase chain reaction; PVA, polyvinyl alcohol; SAF, sodium acetate-acetic acid-formalin; the apicomplexans include Cryptosporidium spp. and the coccidians (Cystoisospora belli and Cyclospora cayetanensis).

POLYVINYL ALCOHOL (PVA) ADHESIVE (NOT A FIXATIVE)

Polyvinyl Alcohol (PVA) is a water-soluble synthetic polymer used as a viscosity-increasing agent in pharmaceuticals, an adhesive in parasitology fecal fixatives, and as a lubricant and protectant in ophthalmic preparations; PVA is also defined as a water-soluble polymer made by hydrolysis of a polyvinyl ester (such as polyvinyl acetate); used in adhesives, as textile and paper sizes, and for emulsifying, suspending, and thickening of solutions. PVA IS NOT A FIXATIVE, BUT AN ADHESIVE TO HELP GLUE THE STOOL MATERIAL ONTO THE SLIDE; this is the only purpose of PVA as an additive to parasitology fecal fixative formulations.

PVA is a plastic resin that is normally incorporated into Schaudinn's fixative. Although some laboratories may perform a fecal concentration from a preserved specimen containing PVA, some parasites will not concentrate well, nor will some exhibit the typical morphology that would be seen in concentration sediment from a formalin-based fixative. Fixatives containing PVA are highly recommended as a means of preserving cysts and trophozoites for later examination as permanent stained smears. The use of fixatives containing PVA also permits specimens to be shipped (by regular mail service) from any location in the world to a laboratory for subsequent examination. Fixatives containing PVA are particularly useful for liquid specimens and should be used in the ratio of 3 parts fixative to 1-part fecal specimen.

NOTE: Very detailed information on all fixative options can be found in:

Garcia, L.S. Diagnostic Medical Parasitology, 6^th ed., ASM Press, Washington D.C., 2016.