Final answer:
Unpreserved sputum should be stored at -70 °C or lower, using an ultra-low freezer or liquid nitrogen tanks to maintain the required temperature.
Step-by-step explanation:
Unpreserved sputum should be stored at ultra-low temperatures of -70 °C or lower for long-term storage or transport. These temperatures help in preserving the biological integrity of the sample and prevent the growth of bacteria that could otherwise alter the results of medical tests. For achieving such low temperatures, specimens are typically stored on dry ice in an ultra-low freezer or within special liquid nitrogen tanks, which can maintain temperatures as low as -196 °C.