Why does hematocrit (Hct) level lag behind 24 to 48 hours after the onset of bleeding?

From the Guidelines

Hematocrit lags 24 to 48 hours from the onset of bleeding because the body's compensatory fluid shift mechanism takes time to equilibrate, and this understanding is crucial for clinicians to accurately assess patients with suspected hemorrhage, as supported by the European guideline on management of major bleeding and coagulopathy following trauma: sixth edition 1. When acute blood loss occurs, the initial response is a proportional loss of both red blood cells and plasma, so the hematocrit (the percentage of blood volume occupied by red blood cells) may remain relatively unchanged at first.

• The body then activates compensatory mechanisms, including fluid shifts from the interstitial space into the vascular compartment to maintain blood volume, a process called transcapillary refill.
• This influx of fluid dilutes the remaining red blood cells, causing the hematocrit to drop.
• Additionally, the renin-angiotensin-aldosterone system activates to promote water and sodium retention by the kidneys, further diluting the blood. These physiological responses take approximately 24-48 hours to fully develop, which explains why hematocrit measurements may not accurately reflect the severity of acute blood loss immediately after bleeding begins, highlighting the importance of repeated Hb and/or Hct measurements as a laboratory marker for bleeding, as recommended by 1. Key points to consider in clinical practice include:
• Initial hematocrit values may be misleadingly normal despite significant blood loss.
• Repeated measurements of hematocrit and hemoglobin are essential for accurate assessment of bleeding severity.
• Understanding the physiological basis of the delayed decrease in hematocrit is vital for timely and appropriate patient management, as emphasized by the guideline 1.

From the Research

Hematocrit Lag in Bleeding

• Hematocrit levels may lag behind the onset of bleeding due to various factors, including the body's compensatory mechanisms and the effects of fluid resuscitation 2, 3.
• When bleeding occurs, the body tries to maintain blood pressure and perfusion of vital organs by constricting blood vessels and increasing heart rate 4.
• Fluid resuscitation, which is often used to treat bleeding, can also affect hematocrit levels by diluting the blood and reducing the concentration of red blood cells 2, 3.
• The type and amount of fluid used for resuscitation can impact the hematocrit level, with colloids potentially causing less dilution than crystalloids 3.
• The lag in hematocrit levels can be attributed to the time it takes for the body to compensate for the blood loss and for the effects of fluid resuscitation to take hold, which can be around 24 to 48 hours 2, 3.

Mechanisms of Hematocrit Lag

• The endothelial glycocalyx layer plays a crucial role in maintaining blood volume and preventing edema, and its degradation can affect the volume-replacement ratio of fluids 3.
• The volume-replacement ratio for crystalloids and colloids can vary depending on the situation, with colloids potentially being more effective in achieving hemodynamic stability in acute bleeding events 3.
• The use of a Bleeding Assessment Tool can help clinicians determine the severity of bleeding symptoms and guide laboratory investigations 5.
• A thorough bleeding, medical, and family history is essential in determining the nature and severity of bleeding symptoms 5.

Clinical Implications

• Understanding the mechanisms of hematocrit lag is crucial in managing patients with bleeding, as it can impact treatment decisions and patient outcomes 2, 4, 3.
• Clinicians should be aware of the potential effects of fluid resuscitation on hematocrit levels and adjust their treatment strategies accordingly 2, 3.
• The use of colloids or crystalloids for fluid resuscitation should be based on the individual patient's needs and the specific clinical situation 2, 3.