Answering your questions

Oct. 19, 2012

Q I have a question about colony counts on wound cultures. We have a new wound care clinic being set up that has requested we perform colony counts on wound swabs. I have found some procedures for this but wonder if there is any standardization. The procedure I have found states that alginated swabs must be used.

A Even though the culture of wound tissue is considered the gold standard, swab cultures are of value in determining treatment. The timing of swab culture is the key factor. It should be collected once there is a clinical diagnosis of wound infection and prior to the initiation of antimicrobial therapy. There are no standardized swab culturing techniques or interpretation of cultures collected by swabbing. To understand the pathology of wound infections and the role of cultures, microbiologists should familiarize themselves by consulting the various journals of the wound, ostomy, continence nurse (WOCN) such as the Journal of WOCN. Cotton or rayon swabs are to be used to collect specimens on pre-cleaned wounds in one of three ways: 1) If the wound is moist, a dry swab is used by rolling in a “Z” pattern from margin to margin, then placed in transport medium, preferably with charcoal. 2) If the wound is dry, the swab should be moistened with sterile saline before collection and transported as stated previously. 3) The swab can be rolled over a 1cm2 area, exerting enough pressure to produce exudate onto the swab.1 The swabs should be cultured by the usual practice in your lab, with enumerations recommended to be grossly reported, e.g., few, moderate, numerous/many, by organism type with susceptibilities. In your specific situation, a collaboration between the wound clinic and the microbiology laboratory should be established to reach an agreement as to collection method, culturing, and reporting that makes the most sense for positive patient outcomes.

–Barbara Strain, MA, SM(ASCP)

Director Supply Chain Analytics

University of Virginia Health System


  1. Cooper R. Ten top tips for taking a wound swab. Wounds International Journal. 2010.


Q Can coagulation testing for PT/INR, APTT and fibrinogen be performed on frozen platelet poor plasma if testing is delayed more than the manufacturer-recommended 8 hours, 4 hours, and 8 hours respectively? The manufacturer does not provide storage temperature recommendations if testing is delayed. For PT, the manufacturer recommends “Store plasma for 8 hours at 20 ±5°C. Do not store plasma at 2°C  to 8°C.” I have read literature and CAP guidelines that make provisions for freezing plasma specimens if testing is delayed. How can I reconcile this issue? Do I need to validate testing post-freezing, or is it sufficient to cite appropriate references in our coagulation SOP?

A Coagulation testing for PT/INR, aPTT, and fibrinogen, along with other coagulation assays, can be performed on frozen specimens. CLSI Guideline H21A5 states that platelet poor specimens removed from the cellular component can be stored for up to two weeks at -20°C. Specimens may be stored longer at -70°C or below for up to 12 months. Most manufacturers refer to the recommended CLSI document in their package insert for specimen collection and preparation. Citing the CLSI approved guideline in your SOP, which includes referenced data, is sufficient for documentation.

–Gary Manuel, BS, MT(ASCP)

Hemostasis Supervisor

University of Virginia Medical Laboratories

Q In our lab, EDTA tubes are used for both CBCs and BNPs. If the EDTA tube is spun and used for the BNP prior to performing the CBC, can it be remixed and used for the CBC?

A The standard practice for performing CBC testing is the use of a well-mixed, whole blood sample using EDTA as the anticoagulant. If plasma has been removed from the tube for BNP testing, the plasma-to-RBC ratio has been altered, thus falsely elevating the CBC results.

–Donna L. Canterbury, MT(ASCP)SH

Hematology Supervisor—Core Laboratory

University of Virginia Health System

Q We currently offer 24-hour urine protein analysis. We have physicians requesting 12-hour collections. Is it acceptable to multiply the 12-hour result by two in order to get a 24-hour result (e.g., mg/24 hours)?

A Due to circadian changes it would not be advisable to use this method to convert a 12-hour urine protein result into a 24-hour urine protein result.  Also, there is a slight difference between results based on whether the patient is upright throughout the collection or supine.

–Judy Hundley, MT(ASCP)

Supervisor, Chemistry Core Lab

University of Virginia Health System

MLO’s “Tips from the clinical experts” column provides practical, up-to-date solutions to readers’ technical and clinical issues from experts in various fields. Readers may send questions to [email protected].