Procalcitonin in the elderly: normal plasma concentrations and response to bacterial infections

Abstract

Bacterial infections are an important cause of morbidity and mortality in the elderly population. The frail elderly, especially those older than 80 years, frequently present with non-specific signs and symptoms of acute infection that present the physician with a diagnostic challenge [1]. A number of acute-phase reactants are used in the diagnosis of acute infections, including leukocyte count, erythrocyte sedimentation rate, and C-reactive protein (CRP) level. Although useful as aids to the clinician, they do not exhibit adequate sensitivity and specificity to be diagnostic markers. One substance that has been widely investigated is procalcitonin (PCT), a glycopeptide consisting of 116 amino acids produced by the C cells of the thyroid gland and a precursor of calcitonin. PCT was first described as an indicator of sepsis and infection in 1993 [2]. The original study found that PCT levels in children with proven bacterial infections were significantly higher than in those with viral infections. These findings have since been corroborated [3]. In previous studies, PCT levels were measured in children and in non-elderly adults. Since only one previous study specifically investigated the usefulness of PCT for identifying illness in the elderly [4], we attempted to define the normal levels of PCT in the healthy elderly population, and to assess the PCT response to acute bacterial infection. Our study group comprised men and women older than 65 years. The control group included 107 independent residents of three homes for the elderly in the Jerusalem area where, according to licensing requirements, routine laboratory testing, including full blood count, erythrocyte sedimentation rate and biochemistry, is performed biannually. An additional amount of blood was drawn for the determination of CRP and PCT levels at the time of routine testing, as determined by the treating physician. Subjects with any complaints suggestive of a recent or acute illness were excluded. The patient group included 80 elderly subjects presenting consecutively to the Shaare Zedek Medical Center in Jerusalem with an acute bacterial infection, as determined by one of the following criteria: clinical presentation of sepsis with positive blood culture, clinical urinary tract infection with positive urine culture, clinical pneumonia with the appearance of a new consolidation on chest radiograph and positive sputum culture, acute cellulitis, or other conditions with clinical and laboratory findings compatible with an acute bacterial infection. All patients had a blood sample drawn for the determination of CRP and PCT levels within 24 h of symptom onset. CRP was measured by nephelometry (Behring Diagnostics, Marburg, Germany), with the results expressed in milligrams per liter. PCT measurement was performed using immune luminometry (LUMItest procalcitonin kit; Brahms Diagnostica, Berlin, Germany) as described previously [5]. Results were expressed in micrograms per liter. The latter method has an analytical assay sensitivity of 0.1 μg/l and a functional assay sensitivity of 0.3 μg/l. Serum samples were collected and stored at −20°C until analysis. The ethics committee of the Shaare Zedek Medical Center approved the study and, in accordance with their directive, blood samples for PCT and CRP were only drawn at the same time that other tests ordered by the treating physician were performed. Data were entered into a computer application based on EPI Info 6.04d (Centers T. Dwolatzky . K. Olshtain-Pops Department of Geriatrics, Shaare Zedek Medical Center, P.O. Box 3235, 91031, Jerusalem, Israel