Sodium Polyanethol Sulfonate, widely known by its acronym SPS, is a specialized additive used in phlebotomy procedures. This chemical compound is found in certain blood collection tubes or culture bottles and plays a unique role distinct from standard anticoagulants. It is incorporated into the collection medium to ensure the integrity and viability of a very specific type of sample. The inclusion of SPS is a deliberate step to modify the collected blood environment, facilitating accurate diagnostic testing in the laboratory. The primary purpose of this additive is to support the detection of microorganisms that may be present in a patient’s bloodstream.
The Chemical Role of SPS
SPS performs a dual function within the blood sample, acting both as a general anticoagulant and a targeted neutralizer of the body’s natural defenses. Like other anticoagulants, SPS prevents the blood from clotting by binding to calcium ions, which are necessary to initiate the coagulation cascade. Maintaining the sample in a liquid state is necessary for proper mixing with the culture broth and for the subsequent automated analysis.
Beyond preventing coagulation, the compound’s more significant role is to counteract the patient’s intrinsic antimicrobial systems. SPS inhibits the process of phagocytosis, where white blood cells would normally engulf and destroy any bacteria present in the blood. It also neutralizes certain humoral factors, such as the complement proteins, which are a group of plasma proteins that can kill bacteria directly. By suppressing these defensive mechanisms, SPS creates a more hospitable environment for infectious organisms to survive and multiply.
Application in Blood Culture Collection
SPS is almost exclusively used in blood culture collection, the laboratory test used to diagnose bloodstream infections like bacteremia or septicemia. The additive is pre-mixed into culture bottles that contain a nutrient broth designed to promote microbial growth. The procedure involves drawing a precise volume of blood and immediately inoculating it into a set of these bottles, typically including one designed for aerobic organisms (which need oxygen) and one for anaerobic organisms (which do not).
The concentration of organisms in the blood of a patient with a bloodstream infection is often very low, making the supportive environment provided by the SPS-containing broth essential. For adult patients, collection guidelines often recommend drawing between 8 to 10 milliliters of blood into each culture bottle to ensure the correct blood-to-broth ratio. SPS is the additive of choice because its unique properties ensure that even small numbers of pathogens are not overwhelmed by the patient’s immune factors, allowing them to grow to detectable levels within the culture medium. The collection process requires meticulous aseptic technique to prevent contamination from skin flora, which would lead to a false-positive result.
Potential Limitations and Interferences
While SPS is generally beneficial for enhancing microbial recovery, it can also interfere with the growth of certain fastidious organisms if its concentration is too high. This inhibitory effect has been specifically observed with bacteria such as Neisseria meningitidis and some anaerobic cocci. If the SPS concentration is excessive relative to the blood volume, it can become toxic to these more sensitive pathogens, potentially leading to a false-negative culture result.
Minimizing Interference
To minimize this interference, maintaining the correct blood-to-broth ratio is paramount during the collection process. The blood culture bottles are engineered to contain a specific amount of SPS-broth medium, and under-filling the bottle will increase the final SPS concentration in the mixture. Some culture systems address this potential issue by incorporating substances like gelatin or activated charcoal into the medium, which can help neutralize the inhibitory effects of SPS on sensitive organisms. This careful balancing of factors ensures that the advantage of SPS in neutralizing immune components outweighs the risk of inhibiting pathogen growth.