Haemoglobin Screening Solution Notes
1. Temperature Variations:
Customers have mentioned that some inspectors have previously told them that the temperature of this Copper Sulphate solution must be well controlled near 25°C (77°F) during the actual use of this solution in Haemoglobin Screening of potential blood donors. This screening method was originally developed and validated by the United States Navy Research Unit at the Hospital of The Rockefeller Institute for Medical Research, New York, in 1949. Quoting from the published work,
"No temperature controls or corrections are needed, because the temperature coefficients of expansion of blood and plasma are so near those of the corresponding Copper Sulphate Solutions that temperature changes between 10°C (50°F) and 40°C (104°F) do not significantly affect the gravity relations."
While this reference validates that the test solution temperatures can vary widely without significant error, the reference does also say that the temperature of the Copper Sulphate screening solution and the temperature of the blood samples should be within 5°C (9°F) of each other at the start of the testing. Details for this validation can be found in the original published work:
"Measurement of Specific Gravities of Whole Blood and Plasma by Standard Copper Sulphate Solutions,"
Phillips, R. A., Van Slyke, D. D., Hamilton, P. B., Dole, V. P., Emerson, K., Jr., and Archibald, R.
M., Journal of Biological Chemistry, 183, March 1950, pp. 305-330.
Quality Assurance Testing
The specific gravity measurement temperature for this Copper Sulphate solution is expected to be 15.6°C (60°F). Specific gravity is temperature dependent. For this Copper Sulphate solution, the measured specific gravity will be lower if the measurement temperature is greater than 15.6°C (60°F) and the measured specific gravity will be higher if the measurement temperature is less than 15.6°C (60°F).
1. Hydrometer Methods
The following are often overlooked sources of error in the hydrometer specific gravity method:
- The AABB Technical Manual says the hydrometer should be calibrated (checked for accuracy) before use. Some (such as typical urine hydrometers) are not accurate enough for precision measurement of the small specific gravity variations encountered.
- The hydrometer may have too wide a specific gravity range on its scale, making accurate estimates from the scale difficult. Use a precision or certified specific gravity hydrometer with an expanded scale covering as small a specific gravity range as possible. Since the reading must be visually estimated, an expanded scale covering a narrow specific gravity range improves the reading accuracy.
- Do not drop the hydrometer into the solution to be tested. Gently lower the hydrometer into the solution until it floats on its own. Drops of solution on the hydrometer stem above the liquid level will cause incorrect results. Do not allow the hydrometer to touch the sides of the solution container during measurement readings.
- Most hydrometer scales (60/60) are accurate only for a temperature of 60°F (15.6°C) for the test solution, and a temperature of 60°F (15.6°C) for the reference high purity water. These 60/60 hydrometer scales will not give an accurate reading at any other temperature. If these 60/60 hydrometers are used for blood bank quality assurance testing of a representative Copper Sulphate test sample at any other temperature other than 15.6°C (60°F), the result will be an incorrect. However, a correction to the reading can be determined by the customer. For the initial hydrometer validation, two or three samples of this product can be tested and approved by the functional monitoring method as described below. These approved Copper Sulphate samples can then be used as standards to determine the corrected reading at the new temperature for each hydrometer intended for routine Quality Assurance testing of future Copper Sulphate lots. These corrected target readings for each Hydrometer can then be used to approve or reject future Copper Sulphate batch samples.
2. Refractometer Methods:
Refractometer methods also suffer from some of the same sources of error as hydrometer methods. This includes incorrect solution measurement temperatures, unless the refractometer is self-temperature-compensating (typically to 20°C), and special refractometer scales for other solutions that are not suitable for Copper Sulphate testing. Suitable lots of these products have a refractive index of about 1.3425 at 20°C, or about 6.5° Brix (%) at 20°C on a Brix refractometer. A urine specific gravity refractometer refractive index scale can be used to determine the refractive index (ND) directly on this Copper Sulphate solution at 20°C. However a urine specific gravity refractometer cannot be used to directly read the specific gravity of this Copper Sulphate solution, since the scale is only accurate and suitable for urine specific gravity testing.
3. Density Methods:
If density (g/mL) of the Copper Sulfate solution is determined at 25°C (77°F), divide the density value by 0.9970 g/mL (the density of water at 25°C) to convert to the specific gravity value.