Background information

Direct microscopic examination (or Direct smear) typically refer to microscopic examination of a specimen before any culture or inoculation is performed.


Can direct smear help detect contamination of clinical samples during sample collection (e.g. contamination of blood culture by skin flora during venipuncture)?

Prior Research

"A situation in which three different morphotypes (cellular types) are seen on direct Gram stain, but more than three organisms are on the culture plate indicates possible contamination." (Bailey & Scott's Diagnostic Microbiology, 13th edition.)

Does this "contamination" refer to contamination in-lab contamination post direct smear, where the number of culture isolates > morphotypes?

Or, does it refer to contamination prior to direct smear, where the contaminant biological count was insignificant to be noticed by microscope but they grew after culturing?

  • I think it would help if you explained your motivation for asking this question.
    – Carey Gregory
    May 6, 2023 at 22:39
  • Alright, thanks for feedback. May 7, 2023 at 17:41

1 Answer 1


It is possible and it would depend highly on what sort of sample you were taking, but in general, yes.

The reasons for this are that a smear on a slide should only (through statistical odds) pick up common organisms, which, assuming you have sampled correctly should be the disease causing ones. Contaminants should, in theory at least, be at a lower abundance, so less likely to be sampled

In addition, because a Gram stain is differential only really for Gram +/- (size can play into it too, but differentiating the species of say Streptococcus is impossible by Gram stain), you can't necessarily tell if two or more species are present in the sample.

For example, say you took a throat swab sample from someone complaining of a sore throat. You put some onto a blood plate and do a quick Gram slide too. On the slide you see small spherical Gram + bacteria. You think great - Streptococcus. However in culture you also see small greyish-white, non-hemolytic colonies and larger white, B-hemolytic colonies. In this example, the grey, non-hemolyitc colonies would be Staphylococcus epidermidis and the hemolytic colonies would be Group A Streptococcus. S. epidermidis is normal skin microflora that wouldn't normally be found in the throat, and is usually only pathogenic around things like ports and lines, so a likely contaminant.

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