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Surfactant protein d /phosphatidyl-choline ratio is increased in tracheal lavage samples from artificially ventilated neonates with sepsis

Presented at the Neonatal Society 2003 Summer Meeting (programme).

Calvert J1, Mackay R-M2, Todd D3, Herbert M1, Postle A3, Wilkinson AR1, Clark HW1,2 (introduced by Howard W Clark)

1 Neonatal Unit, The John Radcliffe, Oxford, UK
2 MRC Immunohistochemistry Unit, Department of Biochemistry, University of Oxford, UK
3 Department of Child Health, University of Southampton, UK

Background: Surfactant protein D (SP-D) is one of the collectin family of proteins which are present in lung and blood. These proteins may play a role in innate immunity and inflammation (1). Premature infants are known to be surfactant deficient, but at present there is no normative data on SP-D levels in neonates and the relationship between collectin levels and infection/ inflammation is unknown.

Aims: To measure SP-D levels in tracheal lavage samples from babies receiving artificial ventilation on the Neonatal Unit at the John Radcliffe Hospital, Oxford and to establish any relationship between gestational age or the presence or absence of sepsis.

Methods: 150 tracheal lavage samples were taken from 74 babies ranging in gestational age from 24 weeks to term. The lavage was carried out according to a standardised protocol in the NNU at the John Radcliffe hospital as part of routine ventilatory care. Specimens were frozen at -20C until being assayed for SP-D levels by sandwich ELISA using antibodies raised against the carbohydrate domain of human SP-D. Recombinant human SP-D was used as a standard and the assay was linear over a range from 10-100ng/ml. Values for SP-D concentration are quoted as ng/ml of lavage fluid. 117 samples were assayed separately for phosphatidylcholine (PC) levels. The values showed a log-normal distribution. Geometric means were calculated and compared using Student's t test. A diagnosis of sepsis was made on the basis of either positive cultures from blood or CSF, or the presence of 2 or more clinical signs initiating treatment for sepsis.

Results: There was no relationship between SP-D levels and gestational age, sex or post-natal age at time of sampling. There was a significant increase in SP-D levels in those babies with a diagnosis of sepsis compared to those without sepsis (p< 0.0001). The geometric mean SP-D level in septic babies was 283.8ng/ml [163.5- 492.8ng/ml] and in non-septic babies was 57.2 ng/ml [42.5-]. PC levels were significantly decreased in babies with sepsis compared to those without sepsis (p = 0.004). The geometric mean PC level was 200.6 nmoles/ml [147.5- 272.8] in non-septic babies and in septic babies was 74.0 nmoles/ml [ 40.0- 137.0 nmoles/ml]. The SP-D / PC ratio was significantly increased in those babies with sepsis compared to those without sepsis (p< 0.0001). The mean SP-D/PC ratio in septic babies was 2.18 [1.06- 4.29] and in non-septic babies was 0.29 [0.21-0.39].

Conclusions: The ratio of Surfactant protein-D to phosphatidylcholine increases significantly in the tracheal lavage samples from artificially ventilated infants with sepsis. This increase appears early in the course and could potentially provide an early marker of septic episodes, consistent with a role for SP-D as an acute phase reactant. Concomitant falls in PC levels may indicate either decreased synthesis of surfactant or increased surfactant degradation in septic preterm infants.

1. Clark, H.W., Reid, K.B. , Sim, R.B. (2000) Microbes Infect. 2: 273-8

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