Blood Culture: Indications, Timing, and Volume

Blood culture is one of the most simple and commonly used investigations to establish the etiology of bloodstream infections (i.e. detection of bacteremia, sepsis, and fungemia). Early diagnosis and accurate identification of the bacteria or fungi causing bloodstream infections provides vital information to clinicians to start appropriate antimicrobial therapy. Providing timely and adequate antimicrobial therapy not only decreases infection-related mortality and cost but also reduces the risk of emergence/spread of drug resistance.

What is a blood culture?

A blood culture is a laboratory test in which blood, taken from the patient, is inoculated into bottles containing appropriate culture media to determine whether infection-causing microorganisms (bacteria or fungi) are present in the patient’s bloodstream.

Aims of blood culture:

• To confirm presence of microorganisms in the bloodstream
• To identify the etiology of the bloodstream infection
• To guide appropriate antimicrobial therapy by providing susceptibility information of isolates

Indications: when should a blood culture be performed?

Blood cultures should always be requested when a bloodstream infection or sepsis is suspected. Some of the most common clinical symptoms in a patient which may lead to a suspicion of a bloodstream infection are:

1. Undetermined fever ( ≥  38°C) or hypothermia ( ≤  36°C).
2. Shock, chills, rigors
3. Severe local infections (meningitis, endocarditis, pneumonia, pyelonephritis, intra-abdominal suppuration etc)

Blood cultures should be collected:

• as soon as possible after the onset of clinical symptoms;
• ideally, prior to the administration of antimicrobial therapy.

If the patient is already on antimicrobial therapy, recovery of microorganisms may be increased by collecting blood sample immediately before administering the next dose and by inoculating the blood into bottles containing specialized antimicrobial neutralization media.

What volume of blood should be collected?

Sufficient blood volume is critical for the successful recovery of organisms causing bloodstream infection. It has been found that most bacteremias in adults have a low number of colony-forming units (CFU) per milliliter (mL) of blood.

More the blood volume cultured greater the chance of isolating pathogen causing bloodstream infection.

Figure: Cumulative sensitivity of blood culture sets

Recommended blood-to-broth-ratio is 1:5 to 1:10, commercial blood culture systems may use a smaller blood-to-broth ratio (<1:5) due to the addition of sodium polyanetholesulfonate (SPS).

Adults

Volume is the single most important variable in blood culture yield, because most adult bacteremias carry very few organisms per milliliter of blood. The rule is simple: about 10 mL of blood per bottle, two bottles per set (one aerobic, one anaerobic), so roughly 20 mL per set. Under-filling a bottle is the commonest reason a true bacteremia is missed.

If anaerobic culture is not available in your setting, use a second aerobic bottle instead of leaving it out. This keeps the total blood volume up, which matters more than the aerobic-anaerobic split.

Pediatric

For infants and children, the recommended blood volume should be based on the weight of the patient, and an aerobic bottle should be used, unless an anaerobic infection is suspected.

It is not safe to take large samples of blood from children, particularly infants. It is safe to obtain as much as 4-4.5% of known total blood volume for culture from infants and children.

Specially formulated culture bottles are commercially available for use in children <2 years of age. They are specifically designed to maintain the usual blood-to-broth ratio (1:5 to 1:10) with smaller blood volumes, and have shown to improve microbial recovery.

For more information refer to table below:

Figure: Blood volumes suggested for cultures from infants and children

Pediatric blood volume is based on the child's total blood volume, not a fixed amount, because a large draw is unsafe in a small child. A common weight-based guide:

The safety ceiling: do not take more than about 4 to 4.5% of the child's total blood volume for culture. Pediatric bottles are formulated to keep the correct blood-to-broth ratio at these smaller volumes.

How many blood culture sets should be collected?

Standards for the timing and number of blood cultures depends on the periodicity of microorganisms in the blood stream (random, intermittent, or continuous).

Collect two sets, or preferably three, for each septic episode, each set from a separate venipuncture site.

• One set alone is not enough, for two reasons. The blood volume is too low, so intermittent bacteremia is missed, and a single positive bottle cannot be told apart from a contaminant.
• Two sets detect the large majority of bacteremias; three sets push detection higher and are standard when endocarditis is suspected.
• Never draw a single set to "screen." If a blood culture is worth doing, it is worth doing as at least two sets.

Drawing each set from a separate site is what makes the contaminant logic work: a skin contaminant tends to appear in only one set, while a true pathogen appears in multiple sets.

Figure: Recommended number of blood culture sets

A contaminant will usually be present in only one bottle of a set of culture bottles, but if it’s a true bloodstream infection, multiple blood culture bottles/set will be positive.

Peripheral vein or central line?

Where the blood is drawn from changes how much you can trust the result.

• A fresh peripheral venipuncture is the preferred source for routine blood cultures. Drawing through an existing catheter picks up organisms colonizing the catheter hub, which raises the contamination rate and can produce a false-positive.
• Draw from a line only when peripheral access genuinely fails, and label the sample as a line draw so the result is interpreted with that in mind.

The one situation where a line draw is not just acceptable but required: suspected catheter-related bloodstream infection. Here you deliberately draw a paired set, one from the catheter and one from a peripheral vein, at the same time.

Differential time to positivity (DTP). Because a colonized catheter carries a heavier bacterial load than the peripheral blood, the catheter bottle turns positive sooner. If the catheter set flags positive 2 hours or more before the peripheral set (with the same organism), that supports a catheter-related bloodstream infection. If both flag at about the same time, the line is probably not the source. DTP is most useful for ruling the line in or out and lets clinicians avoid removing a catheter unnecessarily, though it performs less reliably for S. aureus and Candida, which are often treated by removing the line regardless.

Culturing the catheter tip

When a central venous catheter is removed because a line infection is suspected, the tip itself can be cultured. This is not done by dropping the tip into broth, which cannot separate organisms on the outside of the catheter from true colonization. Instead the standard is the roll-plate (Maki) semiquantitative method: the distal 5 cm of the tip is rolled back and forth across a blood agar plate, and the colonies are counted after incubation.

• 15 or more colony-forming units (CFU) from the tip indicates significant catheter colonization and, with matching blood cultures, supports a catheter-related bloodstream infection.
• Fewer than 15 CFU suggests contamination or insignificant colonization.

The key interpretive rule: a catheter tip culture never stands alone. A positive tip means little without paired blood cultures growing the same organism, because the tip can pick up skin flora during removal. Tip culture and the paired peripheral-plus-line draw are complementary, not alternatives: the tip tells you the catheter was colonized, the blood cultures tell you that colonization reached the bloodstream. Send the tip only when a line infection is genuinely suspected, never as a routine check on every removed line.

Timing of Blood Collection

Timing of blood cultures is not considered a critical factor in determining bloodstream infections, as the diagnostic yield remains the same. Most of the researchers/authors concluded that “the overall volume of blood cultured was more critical in increasing organism yield than was timing”.

Guidelines recommend that the first two/three sets (2 bottles/set) of blood culture be obtained either at one time or over a brief time period (e.g. within 1 hour) from multiple venipuncture sites.

Collection and Dispensing of Blood for Culture

Blood for culture must be collected and dispensed aseptically with great care to avoid contaminating the specimen and culture medium.

1. Using a pressure cuff, locate a suitable vein in the arm. Deflate the cuff while disinfecting the venipuncture site.

2. Wearing gloves, thoroughly disinfect the venipuncture site as follows: Disinfect the venipuncture site with an alcoholic chlorhexidine (chlorhexidine-alcohol) preparation, applied over a circle about 5 cm across and allowed to dry fully. Chlorhexidine-alcohol is the preferred skin antiseptic because it gives lower contamination rates than povidone-iodine and does not need to be wiped off afterward.

   Alternatives are 70% alcohol followed by tincture of iodine (2%). Povidone-iodine also works but must be left to dry in contact for at least 2 minutes, and it is more prone to contamination if that contact time is rushed. Whichever agent is used, let it dry completely and DO NOT TOUCH THE SITE AFTER PREPARATION.

3. Lift back the tape or remove the protective cover from the top of the culture bottle(s). Wipe the top of the bottle using an ethanol-ether swab.
   
   Prior inspection of culture media bottles: Do not use a bottle of culture medium if it shows signs of contamination, i.e. broth appears turbid. Do not use a bottle of thioglycollate broth if it appears oxidized, i.e. more than a third of the top of the medium appears pink when the indicator in the medium is resazurin, or more than 20 mm down from the surface of the medium appears green-blue when the indicator is methylene blue. When oxidation has occurred, the medium must be reduced by steaming before it is used.

4. Using a sterile syringe and needle, withdraw about 20 ml of blood from an adult or about 2-5 mL from a young child.
   Note: When an anaerobic culture is not indicated, collect 10 ml of blood.

5. After removing the needle, if iodine was used, wipe the site with 70% alcohol, since some patients are sensitive to iodine. Chlorhexidine does not need to be removed.

6. Insert the needle through the rubber liner of the bottle cap and dispense 10 ml of blood into the aerobic culture medium bottle. Do not change the needle before injecting blood into the bottle. Changing needles does not lower contamination and only adds a sharps-injury risk.

7. When also culturing for anaerobes, dispense about 10 ml of blood into the anaerobic culture medium bottle. This keeps roughly 10 mL per bottle and about 20 mL per set, matching the adult volume rule.

8. Using a fresh 70% alcohol swab, wipe the top of each culture bottle and replace the tape or protective cover(s). Without delay, mix the blood with the broth. Note: The blood must not be allowed to clot in the culture media because any bacteria will become trapped in the clot.

9. Clearly label each bottle with the name and number of the patient, and the date and time of collection.

10. As soon as possible, incubate the inoculated media. Protect the cultures from direct sunlight until they are incubated.

Transport of Blood Culture Bottles

• Do not refrigerate blood cultures. Generally hold at room temperature until processed, for a maximum of 4 h.
• Refer to the manufacturer’s instructions for the appropriate method to store cultures prior to incubation in automated culture systems.

Rejection criteria

1. Reject blood cultures that are received unlabeled.
2. Do not process if the tube or bottle is cracked or broken.

How to Remember

Volume beats timing. The exact minute you draw matters far less than how much blood you collect. Most bacteremias are low-count, so more blood means more chance of catching an organism. If you remember one thing, remember this.

Two sets minimum, from two sites. One set cannot give enough volume and cannot separate a contaminant from a pathogen. Two sets from two separate venipunctures fixes both problems at once: more blood, and the contaminant-in-one-set-only logic.

Peripheral first; line only for a reason. A fresh peripheral stick is the clean source. Drawing from a line invites hub contamination, unless the line itself is the suspected source, in which case you draw a paired set on purpose.

The line that grows first is the guilty one. In a paired draw, if the catheter bottle turns positive 2 or more hours before the peripheral bottle, the catheter is the likely source. Heavier bacterial load, faster growth.

Don't refrigerate; keep it warm. Blood culture bottles go to the incubator, never the fridge. Cold delays or kills the organisms you are trying to grow. Room temperature, maximum 4 hours, then incubate.

Key exam facts in one table

Where Students Get Confused

"When is the best time to draw, at the fever spike?" Timing is far less important than most people think. Studies consistently show total blood volume drives yield, not the exact moment. Draw promptly and draw enough; do not delay waiting for a spike.

"Why not just draw one big set from one site?" Two reasons. A single set, however large, cannot separate a skin contaminant from a true pathogen, that needs the organism showing up in more than one separately-drawn set. And separate sites reduce the chance that one contaminated stick fools you.

"Drawing from the existing IV line is easier, so why not always do that?" Because the catheter hub is colonized, so a line draw has a higher false-positive rate. A fresh peripheral stick is cleaner. The exception is a deliberate paired draw when the line itself is the suspected source.

"What does differential time to positivity actually tell me?" In a paired draw, if the catheter bottle turns positive well before the peripheral one (2 hours or more), the catheter carried a heavier bacterial load and is the likely source of the infection. It helps decide whether the line must come out.

"Can I refrigerate the bottles if the lab is closed?" No. Blood culture bottles must stay at room temperature and go to the incubator, never the fridge. Cold harms the organisms. Maximum hold is about 4 hours.

References and further readings:

1. CLSI. Principles and Procedures for Blood Cultures. 2nd ed. CLSI document M47. Wayne, PA: Clinical and Laboratory Standards Institute; 2022.
2. Tille PM. Bailey & Scott's Diagnostic Microbiology. 15th ed. St. Louis: Elsevier; 2022.
3. Leber AL, editor. Clinical Microbiology Procedures Handbook. 4th ed. Washington, DC: ASM Press; 2016. DOI: 10.1128/9781683670438.CMPH