Alkaline Phosphatase During Active Menstrual Bleeding
Alkaline phosphatase levels decrease during active menstrual bleeding, reaching their lowest point (trough) in the late luteal phase when menstruation occurs. 1
Cyclic Pattern of ALP During the Menstrual Cycle
ALP follows a natural rhythmic fluctuation throughout the menstrual cycle, peaking in the mid-follicular phase (approximately days 7-10) and declining to its lowest levels during the late luteal phase when menstruation begins. 1
The peak-to-trough difference for alkaline phosphatase averages 8.8 ± 4.0 U/L across the menstrual cycle, representing a clinically measurable variation. 1
This cyclic pattern appears to be mediated by progesterone levels, with ALP showing a significant negative association with progesterone concentrations (P = 1×10⁻¹¹), meaning higher progesterone correlates with lower ALP. 1
Estrogen also negatively influences ALP levels (P = .007), contributing to the decline during the late luteal phase when both hormones are present. 1
Clinical Implications for Laboratory Interpretation
When interpreting ALP values in menstruating women, the phase of the menstrual cycle must be considered, as the same woman can have values that fluctuate above and below normal reference ranges depending on cycle timing. 1
Food and alcohol intake do not modify this hormonal association with ALP fluctuation, indicating the effect is primarily driven by endogenous sex hormone changes. 1
The amplitude of ALP fluctuation varies with demographic factors: it is greater in African Americans and decreases with advancing age. 1
Women with body mass index >30 kg/m² demonstrate smaller fluctuations in liver enzymes, including ALP, suggesting obesity may dampen the hormonal influence on these markers. 1
Important Caveats
This physiologic decrease during menstruation is distinct from the marked elevation of ALP seen in pregnancy (up to 2× upper limit of normal), which is caused by placental production beginning in the second trimester. 2
If ALP is elevated during active menstruation in a woman of reproductive age, this warrants investigation for pathologic causes, as the normal physiologic pattern would be suppression during this phase. 1
The cyclic variation in ALP is modest compared to pathologic elevations—if ALP is >5× upper limit of normal, menstrual cycle effects are unlikely to be the primary explanation and serious pathology must be excluded. 3