Table 1.

Advantages and disadvantages of different NIPD approaches

Technology	Sensitivity/specificity (%)	Cost	Complexity	Reproduced by other groupsa	Advantages	Technical and clinical challenges
Formaldehyde treatment [67,68]	98.2% sensitivity 66.6% specificity	Low	Simple	No	Indirect enrichment of fetal DNA	Requires a large number of informative SNPs
DNA methylation studies using sodium bisulfite [21,78-80]	Small independent studies	Low	Simple	Not tested	Direct enrichment of fetal DNA	DNA degradation, full conversion is rarely achieved
DNA methylation studies using restriction enzymes [22,77,79]	Small independent studies	Low	Simple	Not tested	Indirect enrichment of fetal DNA	Limited to the investigation of regions with restriction sites
Protein-based studies [25,87]	Small independent studies	Low	Simple	Not tested	Direct discrimination of fetal proteins	Requires accurate quantification to distinguish normal from abnormal pregnancies
Next-generation sequencing [19,20,89,91,95]	99.2 to 100% sensitivity 97.9 to 99.7% specificity	High	Complex	Yes	Reliable	Time consuming (more than one week to obtain the result), laborious, requires technical expertise, requires expensive equipment and infrastructure
MeDIP real time qPCR-based approach [98,99]	100% sensitivity 100% specificity	Low	Simple	Not tested	Results obtained within 3 to 4 days	Requires 100% antibody performance, requires extensive quality control of reagents prior to use
Identification of fetal-specific mRNAs [104,106,108,109]	100% sensitivity 89.7% specificity	Low	Simple	Not tested	Direct discrimination of fetal RNA from maternal RNA	Requires a large number of informative SNPs, limited by mRNA stability
Digital PCR-based approach [110,111]	Proof-of-principle	High	Complex	Not tested	Accurate quantification of DNA molecules	Requires technical expertise, simplification is achieved by the use of microfluidic devices, which are expensive and not widely available
Epigenetic-genetic chromosome-dosage approach [78,79]	96.9% sensitivity 92.8% specificity	High	Complex	Not tested	Use of digital PCR	Requires a large number of informative SNPs, simplification is achieved by the use of microfluidic devices, which are expensive and not widely available

^a'No' refers to the presence of published literature indicating failure to reproduce the results by independent groups; 'Not tested' refers to the absence of published literature indicating reproduction of the results by independent groups. MeDIP, methylated DNA immunoprecipitation; PCR, polymerase chain reaction; qPCR, quantitative polymerase chain reaction; SNP, single nucleotide polymorphism.