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Preliminary Tests for Donor Selection

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5. Preliminary Tests for Donor Selection

If the donor is healthy and the physical examination is within normal limits, certain laboratory tests must be done to evaluate suitability of the donor for donation. The tests done are.

* Haemoglobin/haematocrit

I. Haemoglobin estimation

The haemoglobin concentration of a blood donor (male/female should be more than 12.5g/dl.

The haemoglobin may be estimated by the following methods

a. Specific gravity method
b. Haemo Cue method
c. Cyanmethaemoglobin method

a. Specific gravity method

This is widely used as a screening test. It is a simple and rapid procedure and does not require any precision equipment.

Preparation of copper sulfate solution

Dissolve 159.6g of pure air dried crystals of CuSo45l-120 in water and make to l000ml at 25�C. The specific gravity of the solution must be 1100.

This stock solution is diluted to prepare solutions of specific gravity 1052 to 1055 as specified below:

Specific gravity(ml)	Stock solution(ml)	D. Water(g/dl)	H b equivalent
1052	51	49	12
1053	52	48	12.5

				(Minimum Hb)

1054	53	47	13.0	(Minimum Hb)
1055	54	46	13.5

The solutions are stored at room temperature in tightly.capped containers. Method

1. A solutionof specific gravity 1053 is used.

2. The solution is taken in a clean tube and a drop of blood is allowed to fall from a height of 1 cm above the surface of the solution.

The delivered drop breaks through the surface film of the solution and penetrates below the surface.

A. lithe drop is lighter than the test solution it will rise, only a few millimeters, and then sink very slowly indicating a haemoglobin value < 12.Sg/dl.

B. If the drop is of the same specific gravity as the standard test solution, it will become stationery for 10-15 seconds and then fall.

C. If the drop is heavier it will continue to fall during the 10-15 second interval i.e. the haemoglobin is higher than 12.5g/dl.

The solution should be changed everyday or after 25 tests.

b. Haemo Cue method

This is a simple method in which readings are taken In the provided photometer. A microcuvette containing the reagents automatically draws a precise volume of blood from the finger prick. When inserted into the photometer the haemoglobin value is displayed In 15-45 seconds.

Advantages

1. Quick results are available in 1 minute
2. Can be used for mass screening purposes.
3. No precision equipment is required.
4. Uses only a single drop of blood.
c. Cyanmethaemoglobin method

Principle

Blood is mixed with a solution of potassium cyanide and potassium ferricyanide. All forms of haemoglobin except sulphaemoglobin are converted to cyanmethaemogloblin. The absorbance is measured in a colorimeter at a wavelength of 540 nm.

Reagents & Equipment

* Drabkin�s Solution

- Potassium ferricyanide - 200 mg
- Potassium cyanide - 50 mg
- Potassium dihydrogen phosphate - 140 mg
- Water to 1 litre
- pH should be 7.0-7.4

Diluent shoud be clear, pale yellow and must be stored in brown glass bottles at room temperature. Discard if turbid.

* Spectrophotometer
* Test tubes

Procedure

1. Take 5 ml of Drabkin�s solution in a tube.

2. Add 20 ul of blood. Mix well

3. Let stand at room temperature for 10 minutes.

4. Read in spectrophotometer at 540 nm against a reagent blank.

Standard curve

Each laboratory may prepare a standard curve with a reference solution of HICN. The absorbance reading may then be directly related to the haemoglobin concentration.

Advantages

1. All forms of haemoglobin except HbS are readily converted to H1CN.
2. Direct comparison is made with l-IiCN standard and therefore, this method is accurate.
3. Cyanmethaemogloblin is a stable compound and does not deteoriate rapidly.

Precautions

Drabkin�s solution must not be pipetted by mouth as KCN is potentially hazardous.

II. Haematocrit

The estimation of haematocrit can alternatively be used for preliminary screening of blood donors. A haemocrit of > 38% is considered suitable.

It is estimated by the microcentrifuge method. This enables higher centrifugation speeds upto 10,000 g with resultant shorter centrifugation time and superior packing of red cells.

Blood sample used is a capillary sample.

Ill. Blood grouping

It is advisable not to do blood grouping (ABO & Rh) prior to blood donation.

Blood grouping should only be done after blood collection using standard recommended techniques.

Summary

A comprehensive medical history and physical examination usually helps to exclude most of the donors who may be at risk of transmitting infection to the recipient and also ensures donor safety. Confidentiality must be maintained at all stages of the donor selection programme. The blood transfusion centre should create public awareness about safe blood practices and high risk behaviour. They should promote self-deferral by people who are unsuitable as donors.