What is Sahli's method of estimation of hemoglobin?

Sahli's Method of Hemoglobin Estimation - Multiple Choice Questions

What is Sahli's Method?

Sahli's method is a colorimetric technique for hemoglobin estimation that converts hemoglobin to acid hematin using hydrochloric acid, then visually compares the resulting brown color against a standard color scale to determine hemoglobin concentration. This method has been used for decades in resource-limited settings where automated analyzers are unavailable.

Performance Characteristics

Diagnostic Accuracy

• Sahli's method demonstrates high sensitivity (98.2%) but lower specificity (66.2%) when compared to automated analyzers as the gold standard 1
• The method shows good agreement with automated hematology analyzers, with a Kappa score of 0.70 (0.64-0.80) for detecting severe anemia 2
• For severe anemia detection specifically, Sahli's method achieves 84.0% sensitivity and 87.9% specificity 2
• Among children aged 6-59 months, Sahli's hemoglobinometer had 95.5% sensitivity for anemia diagnosis, though specificity was only 70% 3

Comparison with Other Methods

• Sahli's method performs significantly better than the Hemoglobin Colour Scale (HCS) for field hemoglobin estimation, with HCS showing only 30% sensitivity compared to Sahli's 98.2% 1
• Both Sahli's and HCS methods are sensitive but have low specificity in low-prevalence anemia settings, producing high false-positive rates 4
• Sahli's method tends to slightly underestimate hemoglobin values compared to automated analyzers (mean 9.2 g/dl vs 9.4 g/dl) 3

Practical Considerations

Time and Cost Efficiency

• Sahli's method requires 4-5 minutes per test, which is slower than invasive digital hemoglobinometers (2-3 minutes) but faster than some alternatives 3
• The cost per test using Sahli's method is approximately USD 0.25, making it significantly more affordable than HemoCue at USD 4 per test 2
• The method requires trained and skilled technicians for accurate results 3

Clinical Utility

• Sahli's method is most useful for confirming clinically suspected anemia in areas where prevalence is low and hemoglobin levels range from mild to moderate (8-11 g/dl) 4
• The method detects almost all cases of anemia in a given population, even when anemia levels are mild 4
• For targeting blood transfusion decisions in hospitalized children, Sahli's method is more cost-effective than clinical assessment of pallor alone 2

Limitations and Pitfalls

Technical Limitations

• Sahli's method has limited utility for defining anemia prevalence in populations where anemia is moderate to mild, as it often labels healthy individuals as anemic 4
• The method relies on subjective visual color matching, which introduces operator-dependent variability 1
• Neither Sahli's method nor HCS detected any cases of severe anemia (Hb < 7 g/dl) in some field studies 4

Blood Sample Requirements

• Unlike the Hemoglobin Colour Scale, Sahli's method requires more stringent standards for collection, handling, and disposal of blood samples 4
• The method requires capillary blood samples, which necessitate proper training in blood collection techniques 3

Sample MCQs Based on Sahli's Method

1. What is the principle behind Sahli's method of hemoglobin estimation?

• A) Spectrophotometric analysis of oxyhemoglobin
• B) Conversion of hemoglobin to acid hematin and visual color comparison
• C) Cyanmethemoglobin formation
• D) Direct measurement using automated cell counters

Answer: B 1, 2

2. What is the approximate sensitivity of Sahli's method for detecting severe anemia compared to automated analyzers?

• A) 30%
• B) 50%
• C) 70%
• D) 84%

Answer: D 2

3. What is the approximate cost per test for Sahli's method?

• A) USD 0.25
• B) USD 1.00
• C) USD 4.00
• D) USD 10.00

Answer: A 2

4. How long does Sahli's method typically take per test?

• A) 1-2 minutes
• B) 2-3 minutes
• C) 4-5 minutes
• D) 10-15 minutes

Answer: C 3

5. What is a major limitation of Sahli's method in low-prevalence anemia settings?

• A) High cost
• B) Low sensitivity
• C) High false-positive rate
• D) Cannot detect any anemia

Answer: C 4

6. Compared to the Hemoglobin Colour Scale, Sahli's method has:

• A) Lower sensitivity
• B) Higher sensitivity
• C) Equal performance
• D) Cannot be compared

Answer: B 1

7. What type of blood sample is required for Sahli's method?

• A) Venous blood only
• B) Capillary blood
• C) Arterial blood
• D) No blood sample needed

Answer: B 3

8. What is the specificity of Sahli's method for anemia diagnosis in children?

• A) 30%
• B) 50%
• C) 70%
• D) 95%

Answer: C 3

9. In which setting is Sahli's method most appropriate?

• A) High-volume tertiary care laboratories
• B) Resource-limited settings with trained technicians
• C) Home testing by patients
• D) Emergency departments only

Answer: B 3, 4

10. What does Sahli's method tend to do with hemoglobin values compared to automated analyzers?

• A) Overestimate significantly
• B) Slightly underestimate
• C) Provide identical values
• D) Randomly vary

Answer: B 3