Question

1. List the sources of errors in hemoglobinometry.

2. Differentiate oxyhemoglobin, deoxygenated hemoglobin, carboxyhemoglobin and methemoglobin.

3. Give the clinical importance of hemoglobin determination.

Answer 1

1) The source of error may be those of the sample, the method, the equipment, or the operator.

2) Hemoglobin is a protein molecule that binds to oxygen. Hemoglobin forms an unstable, reversible bond with oxygen. In its oxygen-loaded form, it is called oxyhemoglobin and is bright red. In the oxygen-unloaded form it is called deoxyhemoglobin and is purple-blue.

Oxyhaemoglobin is a bright red substance formed by the combination of haemoglobin with oxygen, present in oxygenated blood. Oxyhemoglobin is formed during physiological respiration when oxygen binds to the heme component of the protein hemoglobin in red blood cells.

Hence, when a single heme group in the hemoglobin protein becomes oxygenated, the whole protein changes its shape. In the new shape, it is easier for the other three heme groups to become oxygenated. Thus, the binding of one molecule of O₂ to hemoglobin enhances the ability of hemoglobin to bind more O₂ molecules.

A hemoglobin molecule is composed of a protein group, known as globin, and four heme groups, each associated with an iron atom. In the lungs, each iron atom combines reversibly with a molecule of oxygen. Each hemoglobin molecule also has attached a single cysteine amino acid, which attracts nitric oxide from the lungs.

The other three forms of haemoglobin are present at much lower levels and are:

• Haemoglobin Gower II (α₂ε₂) – Composed of two alpha and two epsilon chains.
• Haemoglobin Portland I (ζ₂γ₂) – Comprised of two zeta and two gamma polypeptides.
• Haemoglobin Portland II (ζ₂β₂) – Made of two zeta and two beta polypeptide chains.

COHb impairs the release of oxygen from Hb by increasing oxygen binding to Hb. The result is a shift of the oxyhemoglobin dissociation curve to the left, which reduces unloading of oxygen in the tissues.

Specifically, the oxyhemoglobin dissociation curve relates oxygen saturation (SO₂) and partial pressure of oxygen in the blood (PO₂), and is determined by what is called "hemoglobin affinity for oxygen"; that is, how readily hemoglobin acquires and releases oxygen molecules into the fluid that surrounds it.

The function of hemoglobin is the transport of oxygen to the tissues from the lungs.

In human blood a trace amount of methemoglobin is normally produced spontaneously, but when present in excess the blood becomes abnormally dark bluish brown. The NADH-dependent enzyme methemoglobin reductase (a type of diaphorase) is responsible for converting methemoglobin back to hemoglobin.

Methemoglobinemia is a condition of elevated methemoglobin in the blood. Symptoms may include headache, dizziness, shortness of breath, nausea, poor muscle coordination, and blue-colored skin (cyanosis). Complications may include seizures and heart arrhythmias.

What are the symptoms of methemoglobinemia?

• headache.
• shortness of breath.
• nausea.
• rapid heart rate.
• fatigue and lethargy.
• confusion or stupor.
• loss of consciousness.

Methylene blue is the primary emergency treatment for documented symptomatic methemoglobinemia. It is given in a dose of 1-2 mg/kg (up to a total of 50 mg in adults, adolescents, and older children) as a 1% solution in IV saline over 3-5 minutes.

3) Hemoglobin is a protein in your red blood cells that carries oxygen to your body's organs and tissues and transports carbon dioxide from your organs and tissues back to your lungs. If a hemoglobin test reveals that your hemoglobin level is lower than normal, it means you have a low red blood cell count (anemia).