Vitamin A, or retinol, is a fat-soluble nutrient required for numerous functions, including cell growth, specialization, and maintaining the integrity of epithelial tissues in the skin, lungs, and gut. It also plays a fundamental role in supporting immune function and is an integral component of the visual cycle. When the body lacks sufficient stores, Vitamin A Deficiency (VAD) occurs, which is a considerable global public health concern, particularly affecting vulnerable populations.
When Is Testing Necessary
A healthcare provider typically considers VAD testing when a patient’s medical history indicates a heightened risk of nutritional inadequacy due to compromised nutrient intake or absorption. High-risk groups include young children and pregnant women, especially those living in areas with limited access to diverse foods.
Individuals with existing conditions that interfere with fat absorption are also candidates for screening. These chronic disorders include Crohn’s disease, celiac disease, cystic fibrosis, and chronic pancreatitis, all of which impair the uptake of fat-soluble vitamins like retinol. Non-specific symptoms such as frequent infections, unexplained fatigue, or poor growth may also prompt a clinician to investigate VAD. The decision to test focuses on establishing the underlying cause.
Clinical and Non-Invasive Assessments
Before resorting to blood work, a provider looks for specific physical signs of VAD, particularly those affecting the eyes. The earliest and most common ocular manifestation is nyctalopia, or night blindness, which is impaired vision in low light. This symptom is a functional sign of insufficient retinol required for rhodopsin production in the retina.
The clinical examination may also reveal xerophthalmia, a spectrum of eye dryness and damage ranging from conjunctival xerosis (dryness of the outer membrane) to corneal ulceration. Another specific sign is the presence of Bitot spots, which are foamy, triangular patches of keratinized epithelium on the white of the eye. Specialized non-invasive tests, such as electroretinography or dark adaptation tests, can functionally confirm a suspected deficiency. These tests measure the retina’s electrical response to light stimuli, objectively assessing the visual system’s capacity to regenerate visual pigments.
The Definitive Laboratory Tests
The primary laboratory method for assessing VAD is measuring Serum Retinol Concentration, which quantifies circulating Vitamin A in the blood. Retinol is transported bound to Retinol Binding Protein (RBP). This test is typically performed using High-Performance Liquid Chromatography (HPLC) for accurate measurement.
A significant limitation of the serum retinol test is that the body tightly controls circulating levels (homeostasis) until the liver’s stores are severely depleted. A normal serum level can therefore mask a marginal deficiency in total reserves. Furthermore, the test is susceptible to acute phase responses; inflammation or infection can temporarily lower RBP levels, reducing serum retinol even if liver stores are adequate.
To gain a more accurate picture of the body’s actual reserves, complex biochemical techniques are sometimes employed. The Relative Dose Response (RDR) or Modified RDR (MRDR) tests indirectly measure stored Vitamin A in the liver. These tests involve administering a small dose of Vitamin A or an analogue and then measuring its appearance in the blood. A rapid surge in the circulating level suggests low liver reserves, indicating an abundance of unbound RBP waiting to pick up the dose.
The most precise method is the Stable Isotope Dilution (SID) technique, though it is invasive and costly. This involves giving the patient a dose of Vitamin A labeled with a non-radioactive, stable isotope. By measuring the dilution of the labeled Vitamin A in the blood after several weeks, researchers can accurately calculate the total body pool and liver reserves. The SID method is considered the gold standard for assessing total Vitamin A status across the entire range of deficiency to excess.
Understanding Your Test Results
Test results from serum retinol measurement are typically reported in micrograms per deciliter or micromoles per liter. Interpretation categorizes the patient’s status into three main groups. A level considered adequate is above 0.70 \(\mu\)mol/L (or 20 \(\mu\)g/dL). Results between 0.35 \(\mu\)mol/L and 0.70 \(\mu\)mol/L suggest a marginal status, indicating insufficient body stores even if clinical signs are absent.
A result below 0.35 \(\mu\)mol/L (or 10 \(\mu\)g/dL) is considered severely deficient and often correlates with overt clinical symptoms like xerophthalmia. A clinician must also factor in the patient’s inflammatory status, as inflammation causes RBP to shift from the blood into tissues. Elevated inflammatory markers like C-Reactive Protein (CRP) can cause a falsely low reading, requiring the provider to adjust the deficiency cutoff or use a different biomarker. Once confirmed, treatment typically involves high-dose Vitamin A supplementation.