From the Guidelines
Postural pseudoanemia is a temporary condition where blood hemoglobin levels appear falsely low when a person is in an upright position compared to when lying down, due to fluid shifts in the body related to gravity. This phenomenon occurs when a person stands up, causing plasma fluid to move from blood vessels into the tissues of the lower extremities due to increased hydrostatic pressure, resulting in a relative increase in hemoglobin concentration in the upper body 1. Conversely, when lying down, fluid redistributes more evenly throughout the body, diluting the blood and causing hemoglobin levels to appear lower. Some key points to consider include:
- This effect is most noticeable in well-hydrated individuals and can lead to misleading laboratory results if blood samples are taken in different postural positions.
- To obtain accurate hemoglobin measurements and avoid misdiagnosis of anemia, blood samples should ideally be collected after the patient has been in a consistent position (preferably sitting) for at least 15-30 minutes.
- Healthcare providers should be aware of this phenomenon when interpreting borderline low hemoglobin results, especially if the patient's clinical presentation doesn't align with anemia symptoms 1. It's also important to note that postural pseudoanemia is not directly related to the provided guidelines for the evaluation and management of patients with syncope, but rather a separate condition that can be influenced by postural changes 1.
From the Research
Definition of Postural Pseudoanemia
- Postural pseudoanemia refers to the posture-dependent change in hematocrit, which can be mistaken for blood loss or acute anemia 2, 3.
- This phenomenon occurs when changes in posture lead to substantial changes in hematocrit, resulting in a relative increase of 11.0%+/-3.6% from lying to standing 2.
Causes and Effects
- The change in posture from supine to upright or vice versa can cause a shift of estimated blood volume of 1 pint, leading to marked changes in hematocrit 3.
- This shift can result in a cascade of unnecessary diagnostic costs, as the changes in hematocrit may be attributed to blood loss or acute anemia rather than the normal physiological response to a change in position 2, 3.
- In hospitalized patients, supine posture can redistribute plasma into the vascular space, leading to dilution of blood constituents and potentially influencing the identification of hospital-acquired anemia 4.
Clinical Implications
- Recognizing postural pseudoanemia is important to avoid unnecessary diagnostic costs and to accurately assess true pathological changes in patients 3.
- Healthcare providers should be aware of the potential impact of posture on hematocrit and hemoglobin levels, particularly in hospitalized patients 4.
- Studies have shown that patients can exhibit a significant increase in hemoglobin levels when changing from a supine to an upright position, with a median increase of 0.60 g/dL and a relative change of 5.2% 4.