It is important to bear in mind that many worm infections may be diagnosed by simple examination of the faeces, sputum, urine, blood or skin. Helminths which produce a large numbers of eggs or larvae are naturally easier to identify than infections with only a few eggs or larvae. In the latter case, it is helpful to enrich the volume of the parasitic material to be examined, by means of concentration techniques. In this way it is possible to make a diagnosis in many patients who have a low wormload.

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The tests mentioned above cannot, however, produce a diagnosis in the following cases:

• Infection with immature parasites. In acute Katayama fever no eggs are found early in the disease.
• Infections with male worms. This is why it is important to know whether or not a parasite is hermaphrodite, e.g. in infections with male Ascaris lumbricoides.
• Infections with adult worms which are located in an enclosed space, such as the brain.
• Infections with larvae where the human is the intermediate host, e.g. cysticercosis, echinococcosis, visceral larva migrans. Trichinellosis may also be included here.
• Infections with old or damaged worms, e.g. after use of antihelminthics.
• Many patients with loasis do not have microfilariae in their blood

Recognition of worm eggs requires training, practice and experience. Otherwise it is possible to interpret a certain microscopic structure wrongly for years (quality control is important).

• Size. Since infections with Ascaris lumbricoides are so common, the size of a fertilised egg (60 µm) can be used as a reference measure. If no special microscopic eyepiece is available to carry out measurements, the relative size of a structure can be compared to a fertilized Ascaris egg.

• Eggs much larger: Fasciola hepatica, Fasciolopsis buski, S. mansoni, S. haematobium
• Eggs somewhat larger: Paragonimus, S. japonicum, Trichostrongylus orientalis, Hymenolepis diminuta, Ascaris unfertilised egg
• Same dimensions: hookworms eggs, Hymenolepis nana, Diphyllobothrium latum
• Eggs somewhat smaller: Trichuris, Enterobius, Taenia solium and T. saginata
• Eggs much smaller: Clonorchis, Metagonimus, Opisthorchis.

• Shape. Most eggs are symmetrical. The exceptions are those of Enterobius, T. orientalis, D. dendriticum and unfertilised Ascaris. The eggs of hermaphrodite trematodes often have an operculum. This small structure is not always easy to see, however. Some other worms also have it (D. latum). Polar caps occur in Trichuris trichiura and Capillaria sp., giving them a lemon-like appearance. Some eggs, such as various schistosomes, have a spine. These may be large or small compared to the egg, and protrude either terminally or laterally.
• Colour. Many eggs have a rather yellowish brown colour due to bile salts. Some are more or less colourless (hyaline), such as those of hookworms, T. orientalis, E. vermicularis and Ascaris (if there is no protein mantel on the egg).
• Egg shell. This may be surrounded by a knobbly protein layer, as in Ascaris. In some worms the egg shell is thin, as in hookworms. In others it is thick, as in lung flukes.