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Surfactant Protein D (SP-D)

Published: July 27, 2026Updated: August 25, 2026

Surfactant Protein D (SP-D) is a protein biomarker that plays a crucial role in the innate immune response within the lungs. It is primarily used to assess lung health and diagnose pulmonary diseases.

Quick summary

SP-D is a biomarker used to evaluate lung health and diagnose pulmonary diseases.

Medically reviewed by WellAlly Editorial TeamLast reviewed August 25, 2026Editorial policy

Key facts at a glance

Units
ng/mL
Clinical utility
diagnostic
Result kind
quantitative
FDA-cleared
Roche SP-D, Yamasa SP-D
interpretation: {"text":"Levels below the reference range may indicate a lack of surfactant production or other underlying conditions affecting lung function.","label":"Low Levels"}; {"text":"Levels above 110 ng/mL may suggest lung inflammation, damage, or disease such as COPD, idiopathic pulmonary fibrosis (IPF), or acute respiratory distress syndrome (ARDS).","label":"High Levels"}

Surfactant Protein D (SP-D) is a hydrophilic protein produced by alveolar type II cells and Clara cells in the lungs. It is a component of pulmonary surfactant, which reduces surface tension in the alveoli and contributes to the lung's defense against pathogens.

Elevated levels of SP-D in the blood can indicate damage to the alveolar-capillary membrane, which is often associated with acute lung injury, chronic obstructive pulmonary disease (COPD), and other inflammatory lung conditions.

Measuring SP-D levels is useful in diagnosing and monitoring the progression of lung diseases, as well as assessing the effectiveness of therapeutic interventions. Normal reference levels typically range from 0 to 110 ng/mL.

Frequently asked questions

What is the role of SP-D in the lungs?
SP-D is part of the pulmonary surfactant and plays a role in the immune response by helping to defend against pathogens and reduce surface tension in the alveoli.
How is SP-D measured?
SP-D levels are measured through a blood test, with normal levels ranging from 0 to 110 ng/mL.
  1. 1
    Surfactant Protein D in Health and DiseaseSource
  2. 2
    Role of SP-D in Pulmonary Host DefenseSource

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This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 25, 2026. · Read our editorial policy