Blood culture

Recommended to obtain 10 mL of blood per culture as a lesser volume decreases the yield of organisms.

Routinely taken when sepsis suspected and can provide information for therapeutic decisions.

Most blood cultures do not have adequate volume of blood.

For each mL less than 10 mL the yield of organisms decrease by approximately 3% (Mermel).

Blood cultures that are positive represent true infections about half of the time.

Two blood cultures drawn from separate sites of the body are often sufficient to diagnose bacteremia.

Two out of two cultures growing the same type of bacteria usually represents a real bacteremia, particularly if the organism that grows is not a common contaminant.

One out of two positive cultures will usually prompt a repeat set of blood cultures to be drawn to confirm whether a contaminant or a real bacteremia is present.

Many blood cultures have coagulase negative staphylococci, which are the most common organisms grown from blood cultures, and 90% are contaminants.

When blood cultures with coagulation negative staphylococci do represent true infections it is usually associated with an intravenous catheter or other prosthetic material, such as a heart valve, a vascular graft or a stent.

The most common organisms causing true bacteremia are Staphylococcus aureus, E. coli is second, enterococci is third and Klebsiella a gram-negative organism is fourth.

Blood cultures should be done in only the sickest patients with community acquired pneumonia, as routine blood cultures for such patients who are hospitalized have limited utility and false-positive results lead to inappropriate antimicrobial use and longer hospital stays.

Routine blood cultures in pneumonia have extremely low yield and usefulness, irrespective of the severity and risk of disease (Zhang D.).

Most laboratories can provide the results of a Gram stain of organisms found in positive blood cultures, either simultaneously or within a few hours.

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