## 8 Future Directions  
According to the above studies, matrix metalloproteinase-1, -13 (MMP-1, -13) and their tissue inhibitors (TIMPs-1, -2) have promising scope favoring future clinical applications of these biomolecules as serological markers for the diagnosis, prognosis, and monitoring of the development of preeclampsia. Despite that, there are some controversial findings and limitations for their routine use. Several important critical remarks make gaps in evidence. Changes in collagen type I and type III turnover are likely to occur at a very early stage. Many studies were undertaken in women with established preeclampsia [56; 57; 59; 62; 64; 65; 66]; therefore, it cannot be determined whether the change in MMPs and TIMPs levels were a cause or a consequence of the disease. To more precisely determine the extent of the association of these markers with concurrent preeclampsia, it would be necessary for healthy pregnant women with family history of preeclampsia to be closely monitored and tested for changes in collagen type I and III turnover biomarkers, prior to preeclampsia development. Different studies examine different patient groups including women with wide range of hypertensive disorders of pregnancy, such as women with gestational hypertension [61; 62; 64; 66] vs. preeclampsia [56; 57; 59; 68]. Moreover, some studies investigate patients with early onset preeclampsia \(\leq\)34 gestational week, while others focus on late onset of preeclampsia \(\geq\)34 gestational week. This makes it very difficult to compare the results. Whether collagen metabolism markers give enough information like lone indicators, or clinicians need a combination model adding other known biomarkers remains to be determined. For example, pregnancy associated plasma protein A (PAPA), placental growth factor (PIGF), or soluble FLT-1 (also known as soluble VEGF receptor 1 or sFLT-1) have been previously explored as preeclampsia diagnostic and prognostic biomarkers. Probably, the complex use of an integrative model combining different types of markers looks more reasonable than just one indicator. Considering the commented investigations in this review, some important differences can be observed between the methods for detection of serum markers of collagen type I and III turnover. Some researchers use zymography for detection of biomolecules' activity [58; 63], other use ELISA--for measuring biomarkers' concentration [57; 59; 61; 62; 65; 66; 68]. Studies [56; 64] use both Zymography and ELISA for diagnosis, prognosis, and monitoring of the development of preeclampsia. Analyzing the above mentioned studies, differences can be observed between the type of samples used for detection of markers of collagen type I and III turnover, serum [57; 66; 68] vs. plasma [56; 61; 62; 63; 67; 65]. Not only absolute MMPs and TIMPs levels are important, but also the relationship between their ratios [57; 61; 63; 65] could play a key role in uterine vascular remodeling in hypertensive disorders of pregnancy. Probably, MMPs/TIMPs ratios should be tested along with MMP and TIMP levels lone. Clinical severity of preeclampsia should also be considered. It is important to take into account mild vs. severe preeclampsia cases. It can influence the concentration of circulating collagen biomarkers. Cross-sectional studies report levels of biomarkers only at one certain point. These markers should however be determined in serial measurements at various time points. This calls the need for larger and longitudinal investigations to be accomplished. Early pregnancy prediction of term preeclampsia remains too poor to be of clinical use. That is why the SCOPE study [67] suggests a concept of 2-stage screening with screening in early pregnancy for early-onset disease and screening for term preeclampsia in the third trimester, despite the fact that it is not fully investigating collagen type Iand III biomarkers. It has not been completely established whether a serum biomarker is representative for the tissue level concentration.