What does a low hematocrit (Hct) level indicate?

What a Low Hematocrit Indicates

A low hematocrit (Hct) primarily indicates anemia, which is a reduction in the proportion of blood volume occupied by red blood cells, and requires further investigation to determine the underlying cause as it can significantly impact morbidity and mortality.

Understanding Hematocrit

Hematocrit is a measure that indicates the proportion of whole blood occupied by red blood cells 1. It is typically expressed as a percentage and varies by:

• Age
• Sex
• Pregnancy status (in women)

Normal hematocrit values range from:

• Adult males/post-menopausal females: 47 ± 6%
• Menstruating females: 41 ± 5%
• Children (varies by age): 35-43% 1

Clinical Significance of Low Hematocrit

A low hematocrit generally indicates:

1. Anemia: The most common interpretation of low hematocrit 1

   • Defined as Hct below the 5th percentile for age, sex, and pregnancy status
   • Reflects a late stage of iron deficiency

2. Potential causes:

   • Iron deficiency (most common)
   • Nutritional deficiencies (folate, vitamin B12)
   • Chronic kidney disease
   • Chronic inflammation
   • Blood loss (acute or chronic)
   • Hemolysis
   • Bone marrow disorders
   • Hemodilution/volume overload 2

3. Increased bleeding risk: Low hematocrit is associated with prolonged bleeding time, independent of platelet count 3, 4

Diagnostic Considerations

Important considerations when interpreting a low hematocrit:

1. Hemoglobin vs. Hematocrit: Hemoglobin measurement is generally preferred over hematocrit because:

   • More reproducible across laboratories
   • Lower coefficients of variation
   • Not affected by storage time or patient variables like serum glucose 1

2. False low readings: Can occur with improper sampling technique (e.g., excessive squeezing/"milking" of finger in capillary samples) 1

3. Hemodilution vs. True Anemia: A low hematocrit may result from:

   • Increased plasma volume (hemodilution)
   • Reduced red blood cell volume (true anemia) 2

4. Confirmatory testing: A single low hematocrit should be confirmed by:

   • Repeat testing
   • Complete blood count
   • Additional iron studies 1, 5

Clinical Approach to Low Hematocrit

When encountering a low hematocrit:

1. Confirm the finding:

   • Obtain a second sample, preferably venous 1
   • Check hemoglobin (more reliable marker)

2. Assess for anemia severity:

   • Compare to age/sex-specific reference ranges
   • Evaluate for symptoms (fatigue, weakness, dyspnea)

3. Determine the cause:

   • Complete blood count with red cell indices
   • Reticulocyte count (to assess bone marrow response)
   • Iron studies (serum ferritin, transferrin saturation)
   • Consider other tests based on clinical suspicion 1

4. Evaluate for true anemia vs. hemodilution:

   • Particularly important in heart failure patients where hemodilution is common and associated with worse outcomes 2

Key Pitfalls in Hematocrit Interpretation

1. Assuming all anemia is iron deficiency: Multiple causes of anemia exist, and proper diagnosis requires additional testing 1

2. Relying solely on hematocrit/hemoglobin: Normal values do not exclude iron depletion, which can quickly progress to deficiency if not detected early 5

3. Failing to consider sample collection issues: Improper collection techniques can cause falsely low readings 1

4. Overlooking hemodilution: In conditions like heart failure, low hematocrit may reflect volume overload rather than true anemia 2

5. Missing bleeding risk: Low hematocrit increases bleeding risk independent of platelet function, which is particularly important in thrombocytopenic patients 3, 4