Bedside procalcitonin and acute care.

Procalcitonin (PCT) is a 116-amino acid protein with a sequence identical to that of the prohormone of calcitonin. Under normal conditions a specific protease cleaves all PCT to calcitonin, katacalcin and an N-terminal residue and hence in healthy individual PCT levels are either too low or undetectable. However, in severe bacterial infections or septic conditions, intact PCT is found in the blood and the concentrations of PCT may reach up to 1000 ng/ml. Point-of-care testing (POCT) is an important diagnostic tool used in various locations in the hospital, especially in intensive care unit (ICU), the operating room (OR), and emergency set-ups. Laboratory test results are often pivotal to fast decisions in majority of areas where patients are critical. Testing provides physicians with valuable knowledge about the emergency in the patients so that appropriate therapeutic interventions can be made quickly. Early detection of rising PCT levels has great significance and helps in diagnosing and managing the patients quickly. This review highlights various facts about PCT in point-of-care scenarios.

INTRODUCTION

Procalcitonin (PCT), 116-amino acid peptide, is the precursor molecule of calcitonin and is devoid of known hormonal activity.[1] Serum levels of PCT are very low in healthy individuals. Despite little being known about the biologic properties of PCT and its origin, PCT levels rise during bacterial infections and other septic conditions. Laboratory testing of PCT has proved helpful in diagnosis of bacterial infections and differentiating it from viral infections. With invention of point of care testing (POCT), these laboratory tests have reached the bedside of patients. Point-of-care testing is defined as medical testing at or near the site of patient care.[2] It has a concept of “testing at or near the site of patient care whenever the medical care is needed”.[2] The driving notion behind POCT is to get the test easily, conveniently and immediately to the patient. This increases the likelihood that the patient, physician, and rest of the team will receive the results quicker that allows for immediate clinical management decisions. POCT includes: Blood glucose testing, blood gas and electrolytes analysis, rapid coagulation testing (PT/INR, Alere®, Microvisk Ltd), rapid cardiac markers diagnostics (TRIAGE, Alere®), PCT, drugs of abuse screening, urine strips testing, pregnancy testing, fecal occult blood analysis, food pathogens screening, hemoglobin diagnostics, infectious disease testing and cholesterol screening. Clinicians may thus have immediate access to battery of targeted laboratory parameters for diagnostic and therapeutic interventions and strategies as well as for predicting the morbidity and mortality in patients admitted in intensive care unit (ICU) and hence benefitting the overall clinical outcome.

The goal is to collect the specimen and obtain the results in a very short period of time at or near the location of the patient, so that the treatment plan can be adjusted as necessary before the patient leaves. Hand-held instruments utilize modern techniques of microfluidics and microsensorics. They can determine one or several parameters quantitatively in different combinations. They have automatic calibration programs with a control system.

PROCALCITONIN

Procalcitonin was first identified from a medullary thyroid carcinoma cell line.[3] It is a 116-amino acid protein with a sequence identical to that of the prohormone of calcitonin.[4] Under normal conditions, a specific protease cleaves all PCT to calcitonin, katacalcin and an N-terminal residue and hence in healthy individual PCT levels are either too low or undetectable. However, in severe bacterial infection and sepsis, intact PCT is found in the blood. In case of severe sepsis, concentrations of PCT may reach up to 1000 ng/ml.

In healthy persons, PCT is synthesized by the C cells of the thyroid. However, in microbial infection there can be alternate pathways. Muller et al. in 2001 described that PCT synthesis can be induced by inflammatory cytokines like interleukin-1 alpha (IL-1 â), tumour necrosis factor alpha (TNF-á) and also by lipopolysaccharides.[5] They could detect mRNA for PCT in all investigated tissues. No specific route of elimination of PCT has yet been established. Probably it is degraded by proteolysis like other plasma proteins. Renal excretion plays a minor role.[6] Half-life of PCT is about 20-24 hours. The tissue of origin of PCT in sepsis has not been confirmed, though there are studies that suggest activated macrophages and hepatocytes might be the possible site of origin. PCT induction is very rapid. Initially the level increases within 2-6 hours, reaching plateau after 6-12 hours. The concentration remains high for up to 48 hours, falling to baseline within the following 2 days.[6]

POCT is often accomplished through the use of transportable, portable, and handheld instrument with test kit. Point-of-care instruments can be classified as “transportable” “portable,” or “handheld,” devices.[7] Transportable and portable devices, also called as table or bench-top instruments, are miniaturized forms of classical laboratory instruments.

POCT MONITORS

Point-of-care testing devices can be used in vitro, ex vivo, or in vivo and include many options. The analyzer traditionally has been defined by laboratory medicine as an in vitro measurement device that requires the permanent removal of blood, fluids, or tissue from the patient. A monitor can be defined as a measurement device that does not require permanent removal of blood, fluid, or tissue from the patient and can be invasive or non-invasive. A blood monitor can be used ex vivo or in vivo. Ex vivo technologies use specimens that are drawn from the patient and analyzed just outside the body, with some of them allowing the return of the specimens to the patient (i.e. intra arterial catheter attached to an analyzer). In vivo technologies provide the opportunity to measure pH, blood gases, electrolytes, lactate, and glucose levels directly, using invasive sensors.

Cheaper, smaller, faster, and smarter POCT devices have increased the use of POCT approach in early diagnosis and implementation of treatment of sepsis and other infections. PCT levels play significant role in early diagnosis of sepsis.

INDICATIONS OF PCT MEASUREMENT

Following are the indications of PCT measurement:

Diagnosis of sepsis

Septic Shock

Differential diagnosis of infectious and non-infectious etiologies of a systemic inflammatory response

Viral and bacterial infections

Infected and sterile pancreatic necrosis

Monitoring of patients at high risk or sepsis and infection.

Many of them require unit-use reagents, which mean that an individual portion of reagent is consumed by a single measurement test.[8] Portable instruments can be easily carried out with a built-in handle. The transportable equipment is usually carried on a cart.[7]

WHAT ARE THE EQUIPMENTS AVAILABLE?

Many instruments or devices are available in recent times for measuring PCT levels to have early diagnosis through POCT. PCT-Q® (Brahms, Germany) is a semi-quantitative rapid immunochromatographic test that uses lateral flow immunochromatography to measure PCT.[9] The test requires 200 μL of serum/plasma obtained from venepuncture and a 30-minute incubation period to complete.[9] At PCT concentrations of ≥0.5 ng/ml, a reddish band forms, the color intensity directly proportional to the PCT concentration.[9] The test is read with a reference card, which allows the patient's PCT level to be classified into one of four semi-quantitative categories (<0.5 ng/ml, 0.5 to 2 ng/ml, 2 to 10 ng/ml and >10 ng/ml).[3] A centrifuge is required to separate the serum/plasma. In clinical practice, PCT values appear to be available within one hour of venepuncture.[4]

ACCURACY COMPARED TO EXISTING TECHNOLOGY

The standard laboratory method of measuring PCT is quantitatively, using Lumitest® (Brahms, Germany), a luminometric immunoassay or Kryptor® (Brahms, Germany), an ultra-sensitive immunoassay using TRACE (Time Resolved Amplified Cryptate Emission) technology. Conversely, PCT-Q® (Brahms, Germany) semi-quantitatively measures PCT values in the serum of patients. The accuracy of the PCT-Q® test varies based on the number of technicians involved: The rate of disagreement (discordance) is low in studies with a limited number of technicians but higher when several technicians are involved.[4]

What are characteristics of contemporary point-of-care instruments:

Able to measure more than 10 different tests per sample of blood

Requires a small blood volume to perform a test (2.5 μL to 40 μL for single to multiple measurements)[7]

Rapid analysis time

Provides selective testing (operator may select the tests to be performed)

Advanced quality management features (automatic internal calibrations at assorted time intervals)

Safety features (like password option).[7]

WHAT ARE ADVANTAGES AND DISADVANTAGES OF THIS TEST AS POCT?

The advantages of PCT as POCT are:

Simpler sample collection

Simpler pre-analytical process

Faster test results available leading to more timely treatment

Removes pathology access barriers

Increased patient satisfaction

Early diagnosis and treatment can be instituted earlier.

Disadvantages

Increased workload

Potential errors due to lack of expertise and quality control

Potentially incompatible to local laboratory method used

Increased cost

Inadequate storage of results

Standardization and calibration.

The laboratory procedure begins with the physician's order for laboratory studies, using testing profiles and prioritization levels (standard, urgent, and stat) pre configured by the laboratory administration. The samples are drawn and transported to the central laboratory for analysis. This multistep and multiperson central laboratory program may lead to long delays between the time a laboratory test is ordered and the time the result is received by the clinical staff and may not keep pace with the dynamically evolving clinical needs of the unstable, critically ill patient.[7] The emergency department, intensive care unit (ICU)/operating room (OR) diagnostic and therapeutic processes are rendered inefficient and ineffective, if the ICU/OR staff finds that test results are classified as “pending” and are not available at the time they are required for decision making. Even when laboratory results arrive at the ICU/OR, they reflect the past and not the patient's present condition. There are certain problems that arise out of the delays involved with traditional laboratory testing and these can definitely change the outcome of the critically sick patients with rapidly changing internal milieu and large volume shifts, in the OR setting and the ICU. Analysis of serum electrolytes is an example, and if reports not available immediately, it will be useless in terms of replacement therapy.

WHAT ARE THE PROBLEMS WITH TRADITIONAL LABORATORY?

Increased therapeutic turn-around time

Delayed data availability

Delayed decision making

Increased blood loss

Increased pre-analytic error

More redundant blood tests.

Newer machines are under development using a compact optical immunosensor, a feasibility assessment for the simultaneous multi-parameter detection of PCT, C-reactive protein (CRP) and TNF-alpha has recently been demonstrated.[5]

ADVANTAGES OVER EXISTING TECHNOLOGY

In primary care patients presenting with an acute infection, it can be difficult to distinguish between a serious bacterial infection (SBI) and a self-limiting illness. In situations of diagnostic uncertainty, and when tests are available, clinicians often measure the white blood cell count (WBC) despite having poor diagnostic accuracy. Other inflammatory markers have been proposed to aid the diagnosis of SBI, such as CRP and PCT.

PCT, protein prohormone of calcitonin, is normally present in low levels in healthy individuals and during viral infections, but rises rapidly during SBI. In addition, improved targeting of antibiotic prescribing by having PCT levels guide treatment decisions that has been evaluated for the management of respiratory tract infections as outpatients. The PCT has many potential applications in primary care, out-of-hours and the emergency department (ED) by potentially excluding SBI and subsequently guiding judicious referral decisions and antibiotic usage. In particular, point-of-care PCT tests may be especially useful in rural settings where extensive laboratory investigations are limited or not available. Point-of-care PCT tests may allow earlier diagnosis of sepsis as several studies have reported that PCT is a better diagnostic marker of sepsis than CRP.[8]

Lower respiratory tract infections (RTI) has high incidence each year.[2] The 2008 NICE guidelines on RTI reported that each year most people have an RTI and one-quarter patients visit their general practitioner as a result of the infection.[6] In addition to a potential benefit on antibiotic prescribing, PCT testing may also improve the recognition of serious infections, especially in children.[3] One of most interesting indications of PCT in clinical practice is the monitoring of critically ill patients with possibility of sepsis. Very high levels of PCT are a characteristic of septic shock and contrast with the reduced levels observed in cardiogenic shock, while PCT cytokines are elevated in both the groups.[10]

The inflammatory reaction is a defence reaction aimed at maintaining the integrity of the organism. This response, which may occur immediately or later, leads to clinical and biological effects and diagnostic procedures and scoring systems based on clinical symptoms and biologic factors have been developed. This response is very large and amounts of many proteins and metabolic products may change considerably. These changes are used as markers of inflammation or infection.[11]

HOW DO WE INTERPRET THE RESULTS WITH PCT POCT?

Various authors have described the cut-offs for PCT in POCT. These are as follows:[12]

<0.5 μg/L –Severe bacterial infection unlikely, Severe sepsis or septic shock unlikely, and locally confined infection cannot be ruled out

0.5-2 μg/L –local bacterial or systemic viral infection possible

2-10 μg/ml –systemic bacterial, fungal or plasmodial infection likely.

Most often severe sepsis or septic shock

In some cases, a non-specific increase is possible after major surgery, severe polytrauma or burn injury, or during prolonged cardiogenic shock

>10 μg/ml – Sepsis or septic shock most likely.

ROLE OF PCT IN SPECIFIC PROBLEMS

PCT levels have been useful in various diseases and conditions.

PCT in multiple organ failure

Serum PCT levels are of interest as a biomarker in patients with multiorgan failure. It increases in patients with bacterial pneumonia and septic shock. To the contrary, data suggest that serum PCT levels are not elevated in patients with viral respiratory tract infection, unless there is a superimposed or coincident bacterial infection. Therefore, serum PCT levels potentially can assist in clinical decisions regarding whether patients with respiratory tract infection would benefit from empirical antibiotic therapy. The detection limit of the assay is 0.1 ng/ml whereas healthy subjects have generally <0.1 ng/ml.[13]

PCT and traumatic brain injury

Cerebrospinal fluid PCT concentration is increased after traumatic brain injury (TBI) and is associated with the initial phase response, specifically IL-6. The physiologic significance of the increase in cerebrospinal fluid (CSF), PCT after severe TBI remains to be determined. The levels of PCT need to be evaluated with further studies.[14]

PCT and burns

Burns has direct correlation with sepsis and it is obvious that percentage of surface area burns has important influence on sepsis. Early diagnosis of sepsis with rising titres of PCT in burns patients (>20% TBSA) is very essential and PCT is an important tool. Although, some authors concluded that PCT has equivalent importance like CRP.[151617]

PCT and tuberculosis

PCT has not been found a useful indicator in the diagnosis of acute pulmonary tuberculosis (PTB) and serum PCT levels lack the necessary sensitivity. These cannot be a substitute for microbiology, clinical and radiological data. PCT can be used for monitoring treatment response and disease activity in cases of tuberculosis. The kinetics of these parameters may elucidate PCT availability.[18]

PCT and Meningococcal sepsis

Both adult and pediatric patients of meningococcal sepsis showed that PCT can be a better prognostic marker than lactate and CRP. It can be used as reliable prognostic marker in septic shock.[19]

PCT and Autoimmune diseases

PCT was shown to be superior to other inflammation markers for diagnosing the systemic infections concomitant with autoimmune diseases. PCT is stimulated in most of the systemic infections but on the other hand one has to bear in mind that serum PCT is not elevated in cases of viral or localized infections. The invasive bacterial, parasitic or fungal infections correlate positively with serum PCT levels.[202122]

PCT and malignant diseases

PCT concentration is a valuable additional parameter for the differential diagnosis for elevated plasma CRP concentrations in patients with malignant diseases. In the context of symptoms and clinical course, it contributes significantly to the exclusion of patients with infection. It facilitates the identification of patients with drug-induced and paraneoplastic elevations in CRP concentration, in whom the delay of chemotherapy because of suspected infection is unnecessary and potentially hazardous.[23]

Furthermore, in patients with an initial elevation of PCT levels due to bacterial infection, subsequent sequential PCT levels can be used to assess the effectiveness and duration of antibiotic therapy.

PCT kinetics over the first 72 hours of critical care provided prognostic information about ICU mortality and in-hospital mortality in patients with confirmed or likely sepsis independent of state-of-the-art initial clinical severity scores in two US settings, thereby extending observations of previous European studies.[7]

Overuse of antibiotics in ICU settings has contributed to the increase in antibiotic-resistant bacteria. Most randomized control trials demonstrate that using serial PCT-based algorithms to guide continuation/cessation of antibiotic therapy reduces the number of days patients are on antibiotics without adverse effects. None has demonstrated a reduction in multidrug resistant (MDR) bacterial infections. In practice, there is limited utility for PCT to guide initiation of antimicrobial therapy because most ICU patients are taking antibiotics at admission. Further, these algorithms are not effective in patients with recent trauma, surgery or other global inflammation. Thus, PCT-based algorithms for guiding antibiotic therapy are limited to guiding secession/continuation of therapy in non-surgical/trauma ICU patients. And even in these populations, serial PCT measurements may be cost prohibitive, if no reduction in MDR infections is observed.[242526]

CONCLUSION

To conclude, POCT of PCT is definitely helpful in sepsis and septic shock and other respiratory diseases as an early marker. It can also be guide for antibiotic therapy or regimen and can be a prognostic indicator for morbidity and mortality. Newer machines can be advancement in this field, where PCT can be used at site to have early diagnosis and early start of management of patients. It helps in diagnosis or prognosis in other diseases, which cannot be underestimated.