Fat-soluble vitamin deficiency or FSV deficiency refers to an abnormally low concentration of vitamins A, D, E or K in the serum. They are more commonly diagnosed in infants and children and can be caused either by conditions interfering with the absorption of fat, chronic conditions, cholestasis or, in adults, malnutrition.
Symptoms related to fat-soluble vitamin deficiency do depend on the severity of the deficiency itself; in cases where a mild insufficiency is present, symptoms may be absent or too mild to observe. Profound FSV deficiency typically leads to a plethora of clinical symptoms and can even threaten an individual's life.
Patients with vitamin A deficiency typically report night blindness and xerophthalmia  . Bitot's spots may be visible due to a collection of keratin in the conjunctiva, and the observation of keratomalacia in pediatric patients requires immediate therapeutic intervention as a medical emergency . Children presenting with vitamin A deficiency also display growth delays and may present with an active infection pertaining to the respiratory, gastrointestinal tract or other systems. Patients may also present with infertility and spontaneous abortions without apparent underlying gynecological pathology, dry skin with hyperkeratotic lesions and dry hair . Vitamin A deficiency also results in anemia, in which case the individual may present with weakness, fatigue and, possibly, pallor.
Vitamin D deficiency is known to cause rickets in children. Pediatric patients affected by rickets present with osphyalgia, pelvic pain or pain the legs, muscle weakness and skeletal deformities, such as:
Amongst the adult population, vitamin D deficiency is primarily responsible for causing osteomalacia and osteoporosis ; patients tend to present with fractures caused by mild forces or even spontaneous fractures as well as muscle hypotonia and persisting pain 
Patients with decreased vitamin E serum levels present with neurological symptoms, such as ataxia, similar to Friedrich's ataxia     . Additional symptoms may include muscle hypotonia, a limited visual field or complete loss of vision, cardiac dysrhythmia and dementia.
The predominant symptom of vitamin K deficiency is hemorrhage which can be observed in any region of the body; petechiae, easy and frequent bruising, hematomas, hematuria or constant bleeding that does not respond to the standard pressure . Infants may also display an incomplete facial, nasal or skeletal development or present with epileptic activity, anemia, vomiting and mucosal bleeding, due to atraumatic intracranial hemorrhaging.
Vitamin A levels in a symptomatic patient can be directly measured via a high-performance liquid chromatography serum retinol study. A concentration lower than 0.7 mg/L in patients under the age of 12 years is deemed pathologically low . A serum RBP study can also be used to approximately evaluate the adequacy of vitamin A in the serum, but its accuracy is lower  . The complete laboratory assessment for a potential vitamin A deficiency also includes an iron panel, biochemical profile, zinc and albumin levels and a complete blood count.
With reference to vitamin K, its suspected deficiency can be diagnosed by measuring des-gamma-carboxy prothrombin (DCP) in the serum, as it is a protein molecule which is known to be elevated in the absence of vitamin K and can be safely used as a marker of vitamin K deficiency and hemorrhage . Serum phylloquinone can also be measured, although the results are not completely accurate; a concentration lower than 0.15 mcg/L implies a deficiency . Patients with vitamin K deficiency also exhibit an increased serum prothrombin time (PT) and a normal activated partial thromboplastin time (aPTT) .
Furthermore, in order to measure the concentration of Vitamin E, serum alpha-tocopherol levels must be obtained. Individuals with hyperlipidemia are evaluated by calculating the alpha tocopherol to lipid ratio. Lastly, a potential vitamin D deficiency is diagnosed by measuring serum 25(OH)D concentration.