Deficiency of Vit. C in infant is best estimated by Vit. C level in –
Leukocyte vitamin C

Leukocyte concentration of vitamin is a better indicator of body store, it may be deficient even in the absence of clinical signs of deficiency.
It is estimated by buffy coat preparation.

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Plasma vitamin C

Plasma ascorbic and levels vary widely and cannot be relied upon to detect early vitamin C deficiency. Urinary vitamin C
Saturation of the tissues with vitamin C can be estimated from the urinary excretion of the vitamin after a test dose of ascorbic acid.
Excretion of 80% of the test dose in the urine within 3-5 hr after parenteral administration is considered to be normal.
In vitamin C deficiency, less than 80% of test dose is excreted.
Plasma
Urinary excretion
Buffy coat estimation
Adrenal cortical Vit. C estimation