Rejection Criteria for Microbiological Specimens

By Acharya Tankeshwar •  Updated: 05/21/21 •  6 min read

Microbiology lab should have a policy that defines conditions in which a specimen is unacceptable for processing. Specimen rejection criteria should be distributed to all clinical practitioners. Requesting physician or nursing in charge should always be notified upon receipt of these specimens before discarding them. Such communications should be documented in the laboratory information systems (LIS) with the date and time.

It may be necessary to process some specimens even though they are not clearly identified if they are difficult to repeat. Such  irretrievable specimens include CSF, surgical fluid/tissue/swabs, post mortem specimens, blood cultures/ tips. A notation regarding improper collection should be added to the final report so that physician is able to determine the validity of the results.

This post outlines some of the basic criteria for rejection of clinical samples for microbiology tests. 

Clerical Errors

  1. Unlabeled or incorrectly labeled specimens: Unlabeled (no identification on the container) or incorrectly labeled specimens (when compared with its accompanying request form) should not be accepted for analysis. When a discrepancy is seen between patient identification on requisition form and specimen container label, request a new specimen or have the requesting physician or nurse correct the error in person in the laboratory (identification of a mislabeled specimen or requisition should not be done over the telephone).
  2. Specimens received without a request form: If a specimen is received without a request form, ask for a correctly completed request. However, if the request form is not received on the same day, do not process the specimen.
  3. Request forms received without any specimen: If a matching specimen is not received by the end of the working day, issue a report stating that no specimen was received with the request form.
  4. Specimens received with a request form that is devoid of any patient demographic details: If a specimen is received with a request form that is devoid of any patient demographic details request for a correctly completed request form. However, if the correctly completed request form is not received on the same day, do not process the specimen.
  5. Missing vital information in the requisition form: If the specimen source or type or requested test name is not noted in the requisition form, call the physician or charge nurse to ascertain the missing information.

Inappropriate Containers/Conditions

Delay in Transit or Improper transport

  1. Specimens received that have been delayed in transit (specimen transport time exceeds 2 hours post collection and the specimen is not preserved): Reject the samples that have been delayed in transit as unsuitable for processing.
  2. The specimen has been transported at the improper temperature: Reject the sample and request a new specimen.

Inappropriate specimens

  1. Dry swab: Notify physician or charge nurse and request new specimen, properly submitted in appropriate transport device. If physician insists dry swab be cultured, note on laboratory record and report with a caveat: “microorganisms recovered may not reflect actual microbiota.”
  2. Foley catheter tip: Processing of specimens like Foley catheter tip may produce information of questionable medical value so the physician should be notified informing that the specimen is not suitable for microbiological analysis. Note rejection on requisition and return.
  3. Unpreserved urine held in the refrigerator for >24 hours: Notify physician or nurse in charge and request new specimen. Return requisition with appropriate comment.
  4. Sputum specimen with 25 < WBC, >10 epithelial cells/lpf: Inform physician or charge nurse that specimen is mostly saliva and is not appropriate for culture. Request a repeat specimen.
  5. 24-h collection of urine or sputum for AFB or fungus culture: Inform physician or nurse that according to laboratory manual, three separate first morning specimens of sputum or of urine are the best samples for analysis; reject 24-h specimens.
  6. Gram stain for Neisseria gonorrhoeae on specimens from cervix, vagina, and anal crypts: As these anatomic locations may harbor nongonococcal Neisseria giving false-positive results, the smear should not be examined. Notify the physician or charge nurse why these smears are unsuitable for processing.
  7. Specimen received for anaerobic culture from a site known to have anaerobes as part of the normal flora such as mouth, vagina, fistula or intestinal contents, samples from ileostomy or colostomy, etc. should not be processed and the physician should be informed accordingly.

Inadequate specimens

Many times laboratory receives insufficient quantity of specimen for testing, in those cases laboratory to seek for additional material or test prioritization. For example;

  1. If only one swab is submitted with multiple requests for various organisms (bacteria, AFB, fungi, virus, ureaplasma, etc.) ask the physician to send the additional samples or to prioritize test requests.
  2. If sputum swabs for AFB or fungal culture are received, notify the physician or charge nurse that the specimen is inadequate in quantity for the isolation of these microorganisms. Request properly collected specimens.

Excess specimens

  1. More than one specimen of urine, stool, sputum, wound, or routine throat specimens submitted on the same day from the same source. Notify the physician or charge nurse that as stated in the laboratory manual only one specimen will be processed per day.

References and further readings

  1. Andrea J. Linscott, 2016. 2.1. Collection, Transport, and Manipulation of Clinical Specimens and Initial Laboratory Concerns, Clinical Microbiology Procedures Handbook, 4th Edition. ASM Press, Washington, DC. doi: 10.1128/9781683670438.CMPH.ch2.1
  2. Procop, G. W., & Koneman, E. W. (2016). Koneman’s Color Atlas and Textbook of Diagnostic Microbiology (Seventh, International edition). Lippincott Williams and Wilkins.
  3. Tille, P. (2017). Bailey & Scott’s Diagnostic Microbiology (14 edition). Mosby.

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please email at

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