Table 4. Stability of PQ. Data represent mean (± SD).

Conditions	Untreated***	treated	% remained	n
Plasma, glass container, 4°C, 16 hr
30 ng/mL	29.8±2.3	30.2±1.1	101	3
800 ng/mL	796±4	832±14	105	3
Autosampler plastic vials, 21–25°C, 7 days
30 ng/mL	32.8±1.7	29.4±1.1	89.6	4
800 ng/mL	865±27	857±19	99.1	4
Five freeze-thaw cycles
30 ng/mL	31.7±1.3	30.9±1.2	97.5	3
800 ng/mL	844±12	843±12	99.9	3
Plasma, -70°C, 21 months*
3 ng/mL	2.76±0.15	2.78±0.04	101	3
200 ng/mL	200±6	209±5	104	3
Stock, -70°C, 40 months**
	8170±605	8091±338	99	3

* Measured with our previously published method

**measured with UPLC-PDA with peek tubing injection needle.

*** untreated samples were prepared fresh in plastic vials.