2.2 Specimen Collection, Sterile Technique, and Blood Cultures

2.2 Specimen Collection, Sterile Technique, and Blood Cultures

Quick Answer: Use sterile percutaneous technique for blood and other normally sterile sites, collect enough blood volume in two or more peripheral sets after dry chlorhexidine-alcohol prep, and do not treat a single CoNS or Corynebacterium bottle as true bacteremia. Reject saliva, surface wound swabs, and the wrong urine fraction.

Specimen collection is the rest of outline I.A and a large share of the 10–15% preanalytic slice of the M(ASCP) exam. The exam is not asking you to recite nursing policy. It is asking whether the organism that grows is from the infection or from the collection. That distinction starts with sterile versus clean technique and with whether the site normally has flora.

Sterile technique versus clean collection

Sterile (aseptic) collection is required when the site is normally sterile and any skin or mucosal organism is a contaminant until proven otherwise: blood, CSF, joint fluid, pleural/peritoneal/pericardial fluid, bone biopsy, and deep surgical tissue. Skin antisepsis, sterile gloves, and a closed sterile container are not optional theater. Palpating the vein again with a non-sterile finger after chlorhexidine has dried re-contaminates the site.

Clean collection is used when the site already has flora: stool, throat, expectorated sputum, vaginal swab, and many wounds. The goal is to reduce contaminating flora and to sample the lesion, not to pretend the specimen is sterile. Over-interpreting mixed oral or skin organisms from these sites is as wrong as under-interpreting S. aureus from blood.

Blood culture collection: the highest-yield M skill

Skin antisepsis

Adult and pediatric patients older than about two months: chlorhexidine gluconate in alcohol, applied with friction for about 30 seconds and allowed to dry (another ~30 seconds). Wet chlorhexidine is not antiseptic yet. Povidone-iodine or tincture of iodine is an acceptable alternative when chlorhexidine is contraindicated, but iodine must dry (often 1.5–2 minutes). Seventy percent alcohol alone is weaker. Do not wipe the dried antiseptic off. Do not palpate after prep unless the gloved finger is still sterile. Infants younger than two months often use iodine or alcohol per neonatal policy because of chlorhexidine skin concerns; the exam still wants a dry, timed prep, not a quick alcohol swipe.

Volume, sets, and timing

Volume is the main driver of sensitivity in adults. Typical adult inoculation is 8–10 mL per bottle. One set is usually one aerobic plus one anaerobic bottle (about 20 mL). At least two sets from two different peripheral sites (about 40 mL) are the standard for acute sepsis; three sets (about 60 mL) are used when endocarditis is in the differential. A single 10 mL draw into one bottle is an inadequate adult collection: yield is lower and a contaminant cannot be compared across bottles.

Pediatric volume is constrained by total blood volume. Use pediatric bottles for small draws. A common teaching range is about 1–3 mL in an infant bottle, scaling toward 1 mL per year of age or roughly 2–4% of blood volume, never to the point of iatrogenic anemia. Putting 1 mL into an adult 10 mL bottle underfills the bottle, distorts the SPS-to-blood ratio, and drops yield. Do not "top up" a pediatric draw with saline.

Draw before antibiotics when the clinical situation allows. Do not wait hours for a fever spike in acute sepsis; continuous or intermittent bacteremia is captured by adequate volume from two sites, not by calendar timing. Subacute endocarditis still uses three sets, sometimes separated over time if the patient is stable. Changing needles between skin and bottle is no longer recommended (sharps injury without a contamination benefit).

Peripheral versus catheter draws

Routine blood cultures are peripheral venipuncture. Drawing through an existing central line, PICC, or arterial line raises contamination with CoNS, Corynebacterium, Bacillus species (not B. anthracis), Cutibacterium acnes, and Micrococcus. If catheter-related bloodstream infection is the question, collect a paired peripheral set plus a set from the catheter, labeled as such. Differential time to positivity of ≥2 hours (catheter bottle positive first, same organism) supports CRBSI. A line-only draw that grows CoNS is almost uninterpretable.

Contamination versus true bacteremia

A single CoNS-positive bottle out of four, with three negatives, is probable contamination on the M exam unless the patient has a prosthetic valve, a ventricular assist device, or other hardware and multiple sets are positive with the same organism. Corynebacterium in one aerobic bottle from a peripheral draw is the same story: skin, not endocarditis. S. aureus, gram-negative rods, and Candida are not explained away as contaminants because only one bottle flagged.

Target contamination rates are typically <3%. Poor skin prep, line draws, and underfilled sets all inflate that number and waste the rest of the workup.

Sputum versus saliva

Expectorated sputum must come from the lower airway. Coach a deep cough; first-morning specimens are often richer. In the laboratory, a Gram stain quality screen separates sputum from spit. Many laboratories reject specimens with >10 squamous epithelial cells per low-power field (some use >25) and few neutrophils. Watery, bubbly material with sheets of squamous cells is saliva. Plate it and you will grow viridans streptococci, Neisseria spp., Haemophilus from the oropharynx, Candida, and enteric gram-negatives that colonized the mouth — then someone will treat them as pneumonia.

Induced sputum, tracheal aspirates, and bronchoalveolar lavage are different specimen types with different quantitative rules; do not apply a saliva screen to a labeled BAL. Conversely, do not "save" a salivary specimen by calling it an induced sputum because the order said pneumonia.

Wounds: deep tissue versus surface swab

A dry swab rolled over the surface of a chronic ulcer is a flora catalog. Staph, diphtheroids, Pseudomonas, enterococci, and Candida all live on open skin. The useful specimen is collected after the surface is cleaned: aspirate of pus, curettage of the advancing margin, or tissue from debridement, submitted in a sterile cup or anaerobic transport vial. Label the anatomic site and collection method (left diabetic foot tissue, not "wound swab"). Anaerobes will not survive a swab waving in room air on the way to the lab; if the surgeon wants Bacteroides or clostridia, they must send tissue or an anaerobic vial, not a cotton swab in a dry tube.

Urine: first-void versus midstream

These two fractions are not interchangeable, and the exam loves the swap.

• Midstream clean-catch (or a freshly placed catheter specimen from the port, never the bag) is for UTI culture. Periurethral flora is voided first; the midstream aliquot better represents bladder urine. Colony counts and mixed-flora interpretation assume this collection.
• First-void urine (the first 10–20 mL of the stream, typically without cleaning) is for genital-pathogen NAAT (N. gonorrhoeae, C. trachomatis, sometimes Mycoplasma genitalium). Those organisms live in the urethra; midstream dilutes them and can cause false-negative NAAT.

Straight catheterization and suprapubic aspirates are sterile-site collections and are interpreted more strictly than voided urine. A Foley bag dip is a reject.

Throat and stool, briefly

Throat swabs for streptococcal culture or NAAT sample the tonsillar pillars and posterior pharynx, not the tongue or buccal mucosa. Stool for bacterial culture or C. difficile must be feces, not rectal swab residue, except in infants or when a swab is the only possible sample and the SOP allows it. Formed stool is rejected for C. difficile.

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